diff --git a/packages/babel-helpers/src/helpers.js b/packages/babel-helpers/src/helpers.js
index 4c3c896fcc..79f5255096 100644
--- a/packages/babel-helpers/src/helpers.js
+++ b/packages/babel-helpers/src/helpers.js
@@ -1213,23 +1213,430 @@ helpers.decorate = helper("7.1.5")`
 
   */
 
+  /*::
+
+  type Placements = {
+    static: Key[],
+    prototype: Key[],
+    own: Key[],
+  };
+
+  */
+
   // ClassDefinitionEvaluation (Steps 26-*)
   export default function _decorate(
     decorators /*: ClassDecorator[] */,
     factory /*: ClassFactory */,
     superClass /*: ?Class<*> */,
+    mixins /*: ?Array<Function> */,
   ) /*: Class<*> */ {
+    var api = _getDecoratorsApi();
+    if (mixins) {
+      for (var i = 0; i < mixins.length; i++) {
+        api = mixins[i](api);
+      }
+    }
+
     var r = factory(function initialize(O) {
-      _initializeInstanceElements(O, decorated.elements);
+      api.initializeInstanceElements(O, decorated.elements);
     }, superClass);
-    var decorated = _decorateClass(
+    var decorated = api.decorateClass(
       _coalesceClassElements(r.d.map(_createElementDescriptor)),
       decorators,
     );
 
-    _initializeClassElements(r.F, decorated.elements);
+    api.initializeClassElements(r.F, decorated.elements);
 
-    return _runClassFinishers(r.F, decorated.finishers);
+    return api.runClassFinishers(r.F, decorated.finishers);
+  }
+
+  function _getDecoratorsApi() {
+    _getDecoratorsApi = function() {
+      return api;
+    };
+
+    var api = {
+      elementsDefinitionOrder: [["method"], ["field"]],
+
+      // InitializeInstanceElements
+      initializeInstanceElements: function(
+        /*::<C>*/ O /*: C */,
+        elements /*: ElementDescriptor[] */,
+      ) {
+        ["method", "field"].forEach(function(kind) {
+          elements.forEach(function(element /*: ElementDescriptor */) {
+            if (element.kind === kind && element.placement === "own") {
+              this.defineClassElement(O, element);
+            }
+          }, this);
+        }, this);
+      },
+
+      // InitializeClassElements
+      initializeClassElements: function(
+        /*::<C>*/ F /*: Class<C> */,
+        elements /*: ElementDescriptor[] */,
+      ) {
+        var proto = F.prototype;
+
+        ["method", "field"].forEach(function(kind) {
+          elements.forEach(function(element /*: ElementDescriptor */) {
+            var placement = element.placement;
+            if (
+              element.kind === kind &&
+              (placement === "static" || placement === "prototype")
+            ) {
+              var receiver = placement === "static" ? F : proto;
+              this.defineClassElement(receiver, element);
+            }
+          }, this);
+        }, this);
+      },
+
+      // DefineClassElement
+      defineClassElement: function(
+        /*::<C>*/ receiver /*: C | Class<C> */,
+        element /*: ElementDescriptor */,
+      ) {
+        var descriptor /*: PropertyDescriptor */ = element.descriptor;
+        if (element.kind === "field") {
+          var initializer = element.initializer;
+          descriptor = {
+            enumerable: descriptor.enumerable,
+            writable: descriptor.writable,
+            configurable: descriptor.configurable,
+            value: initializer === void 0 ? void 0 : initializer.call(receiver),
+          };
+        }
+        Object.defineProperty(receiver, element.key, descriptor);
+      },
+
+      // DecorateClass
+      decorateClass: function(
+        elements /*: ElementDescriptor[] */,
+        decorators /*: ClassDecorator[] */,
+      ) /*: ElementsFinishers */ {
+        var newElements /*: ElementDescriptor[] */ = [];
+        var finishers /*: ClassFinisher[] */ = [];
+        var placements /*: Placements */ = {
+          static: [],
+          prototype: [],
+          own: [],
+        };
+
+        elements.forEach(function(element /*: ElementDescriptor */) {
+          this.addElementPlacement(element, placements);
+        }, this);
+
+        elements.forEach(function(element /*: ElementDescriptor */) {
+          if (!_hasDecorators(element)) return newElements.push(element);
+
+          var elementFinishersExtras /*: ElementFinishersExtras */ = this.decorateElement(
+            element,
+            placements,
+          );
+          newElements.push(elementFinishersExtras.element);
+          newElements.push.apply(newElements, elementFinishersExtras.extras);
+          finishers.push.apply(finishers, elementFinishersExtras.finishers);
+        }, this);
+
+        if (!decorators) {
+          return { elements: newElements, finishers: finishers };
+        }
+
+        var result /*: ElementsFinishers */ = this.decorateConstructor(
+          newElements,
+          decorators,
+        );
+        finishers.push.apply(finishers, result.finishers);
+        result.finishers = finishers;
+
+        return result;
+      },
+
+      // AddElementPlacement
+      addElementPlacement: function(
+        element /*: ElementDescriptor */,
+        placements /*: Placements */,
+        silent /*: boolean */,
+      ) {
+        var keys = placements[element.placement];
+        if (!silent && keys.indexOf(element.key) !== -1) {
+          throw new TypeError("Duplicated element (" + element.key + ")");
+        }
+        keys.push(element.key);
+      },
+
+      // DecorateElement
+      decorateElement: function(
+        element /*: ElementDescriptor */,
+        placements /*: Placements */,
+      ) /*: ElementFinishersExtras */ {
+        var extras /*: ElementDescriptor[] */ = [];
+        var finishers /*: ClassFinisher[] */ = [];
+
+        for (
+          var decorators = element.decorators, i = decorators.length - 1;
+          i >= 0;
+          i--
+        ) {
+          // (inlined) RemoveElementPlacement
+          var keys = placements[element.placement];
+          keys.splice(keys.indexOf(element.key), 1);
+
+          var elementObject /*: ElementObjectInput */ = this.fromElementDescriptor(
+            element,
+          );
+          var elementFinisherExtras /*: ElementFinisherExtras */ = this.toElementFinisherExtras(
+            (0, decorators[i])(elementObject) /*: ElementObjectOutput */ ||
+              elementObject,
+          );
+
+          element = elementFinisherExtras.element;
+          this.addElementPlacement(element, placements);
+
+          if (elementFinisherExtras.finisher) {
+            finishers.push(elementFinisherExtras.finisher);
+          }
+
+          var newExtras /*: ElementDescriptor[] | void */ =
+            elementFinisherExtras.extras;
+          if (newExtras) {
+            for (var j = 0; j < newExtras.length; j++) {
+              this.addElementPlacement(newExtras[j], placements);
+            }
+            extras.push.apply(extras, newExtras);
+          }
+        }
+
+        return { element: element, finishers: finishers, extras: extras };
+      },
+
+      // DecorateConstructor
+      decorateConstructor: function(
+        elements /*: ElementDescriptor[] */,
+        decorators /*: ClassDecorator[] */,
+      ) /*: ElementsFinishers */ {
+        var finishers /*: ClassFinisher[] */ = [];
+
+        for (var i = decorators.length - 1; i >= 0; i--) {
+          var obj /*: ClassObject */ = this.fromClassDescriptor(elements);
+          var elementsAndFinisher /*: ElementsFinisher */ = this.toClassDescriptor(
+            (0, decorators[i])(obj) /*: ClassObject */ || obj,
+          );
+
+          if (elementsAndFinisher.finisher !== undefined) {
+            finishers.push(elementsAndFinisher.finisher);
+          }
+
+          if (elementsAndFinisher.elements !== undefined) {
+            elements = elementsAndFinisher.elements;
+
+            for (var j = 0; j < elements.length - 1; j++) {
+              for (var k = j + 1; k < elements.length; k++) {
+                if (
+                  elements[j].key === elements[k].key &&
+                  elements[j].placement === elements[k].placement
+                ) {
+                  throw new TypeError(
+                    "Duplicated element (" + elements[j].key + ")",
+                  );
+                }
+              }
+            }
+          }
+        }
+
+        return { elements: elements, finishers: finishers };
+      },
+
+      // FromElementDescriptor
+      fromElementDescriptor: function(
+        element /*: ElementDescriptor */,
+      ) /*: ElementObject */ {
+        var obj /*: ElementObject */ = {
+          kind: element.kind,
+          key: element.key,
+          placement: element.placement,
+          descriptor: element.descriptor,
+        };
+
+        var desc = {
+          value: "Descriptor",
+          configurable: true,
+        };
+        Object.defineProperty(obj, Symbol.toStringTag, desc);
+
+        if (element.kind === "field") obj.initializer = element.initializer;
+
+        return obj;
+      },
+
+      // ToElementDescriptors
+      toElementDescriptors: function(
+        elementObjects /*: ElementObject[] */,
+      ) /*: ElementDescriptor[] */ {
+        if (elementObjects === undefined) return;
+        return toArray(elementObjects).map(function(elementObject) {
+          var element = this.toElementDescriptor(elementObject);
+          this.disallowProperty(elementObject, "finisher", "An element descriptor");
+          this.disallowProperty(elementObject, "extras", "An element descriptor");
+          return element;
+        }, this);
+      },
+
+      // ToElementDescriptor
+      toElementDescriptor: function(
+        elementObject /*: ElementObject */,
+      ) /*: ElementDescriptor */ {
+        var kind = String(elementObject.kind);
+        if (kind !== "method" && kind !== "field") {
+          throw new TypeError(
+            'An element descriptor\\'s .kind property must be either "method" or' +
+              ' "field", but a decorator created an element descriptor with' +
+              ' .kind "' +
+              kind +
+              '"',
+          );
+        }
+
+        var key = toPropertyKey(elementObject.key);
+
+        var placement = String(elementObject.placement);
+        if (
+          placement !== "static" &&
+          placement !== "prototype" &&
+          placement !== "own"
+        ) {
+          throw new TypeError(
+            'An element descriptor\\'s .placement property must be one of "static",' +
+              ' "prototype" or "own", but a decorator created an element descriptor' +
+              ' with .placement "' +
+              placement +
+              '"',
+          );
+        }
+
+        var descriptor /*: PropertyDescriptor */ = elementObject.descriptor;
+
+        this.disallowProperty(elementObject, "elements", "An element descriptor");
+
+        var element /*: ElementDescriptor */ = {
+          kind: kind,
+          key: key,
+          placement: placement,
+          descriptor: Object.assign({}, descriptor),
+        };
+
+        if (kind !== "field") {
+          this.disallowProperty(elementObject, "initializer", "A method descriptor");
+        } else {
+          this.disallowProperty(
+            descriptor,
+            "get",
+            "The property descriptor of a field descriptor",
+          );
+          this.disallowProperty(
+            descriptor,
+            "set",
+            "The property descriptor of a field descriptor",
+          );
+          this.disallowProperty(
+            descriptor,
+            "value",
+            "The property descriptor of a field descriptor",
+          );
+
+          element.initializer = elementObject.initializer;
+        }
+
+        return element;
+      },
+
+      toElementFinisherExtras: function(
+        elementObject /*: ElementObject */,
+      ) /*: ElementFinisherExtras */ {
+        var element /*: ElementDescriptor */ = this.toElementDescriptor(
+          elementObject,
+        );
+        var finisher /*: ClassFinisher */ = _optionalCallableProperty(
+          elementObject,
+          "finisher",
+        );
+        var extras /*: ElementDescriptors[] */ = this.toElementDescriptors(
+          elementObject.extras,
+        );
+
+        return { element: element, finisher: finisher, extras: extras };
+      },
+
+      // FromClassDescriptor
+      fromClassDescriptor: function(
+        elements /*: ElementDescriptor[] */,
+      ) /*: ClassObject */ {
+        var obj = {
+          kind: "class",
+          elements: elements.map(this.fromElementDescriptor, this),
+        };
+
+        var desc = { value: "Descriptor", configurable: true };
+        Object.defineProperty(obj, Symbol.toStringTag, desc);
+
+        return obj;
+      },
+
+      // ToClassDescriptor
+      toClassDescriptor: function(
+        obj /*: ClassObject */,
+      ) /*: ElementsFinisher */ {
+        var kind = String(obj.kind);
+        if (kind !== "class") {
+          throw new TypeError(
+            'A class descriptor\\'s .kind property must be "class", but a decorator' +
+              ' created a class descriptor with .kind "' +
+              kind +
+              '"',
+          );
+        }
+
+        this.disallowProperty(obj, "key", "A class descriptor");
+        this.disallowProperty(obj, "placement", "A class descriptor");
+        this.disallowProperty(obj, "descriptor", "A class descriptor");
+        this.disallowProperty(obj, "initializer", "A class descriptor");
+        this.disallowProperty(obj, "extras", "A class descriptor");
+
+        var finisher = _optionalCallableProperty(obj, "finisher");
+        var elements = this.toElementDescriptors(obj.elements);
+
+        return { elements: elements, finisher: finisher };
+      },
+
+      // RunClassFinishers
+      runClassFinishers: function(
+        constructor /*: Class<*> */,
+        finishers /*: ClassFinisher[] */,
+      ) /*: Class<*> */ {
+        for (var i = 0; i < finishers.length; i++) {
+          var newConstructor /*: ?Class<*> */ = (0, finishers[i])(constructor);
+          if (newConstructor !== undefined) {
+            // NOTE: This should check if IsConstructor(newConstructor) is false.
+            if (typeof newConstructor !== "function") {
+              throw new TypeError("Finishers must return a constructor.");
+            }
+            constructor = newConstructor;
+          }
+        }
+        return constructor;
+      },
+
+      disallowProperty: function(obj, name, objectType) {
+        if (obj[name] !== undefined) {
+          throw new TypeError(objectType + " can't have a ." + name + " property.");
+        }
+      }
+    };
+
+    return api;
   }
 
   // ClassElementEvaluation
@@ -1264,8 +1671,8 @@ helpers.decorate = helper("7.1.5")`
       placement: def.static
         ? "static"
         : def.kind === "field"
-          ? "own"
-          : "prototype",
+        ? "own"
+        : "prototype",
       descriptor: descriptor,
     };
     if (def.decorators) element.decorators = def.decorators;
@@ -1292,7 +1699,9 @@ helpers.decorate = helper("7.1.5")`
   ) /*: ElementDescriptor[] */ {
     var newElements /*: ElementDescriptor[] */ = [];
 
-    var isSameElement = function(other /*: ElementDescriptor */) /*: boolean */ {
+    var isSameElement = function(
+      other /*: ElementDescriptor */,
+    ) /*: boolean */ {
       return (
         other.kind === "method" &&
         other.key === element.key &&
@@ -1351,365 +1760,6 @@ helpers.decorate = helper("7.1.5")`
     );
   }
 
-  // InitializeClassElements
-  function _initializeClassElements /*::<C>*/(
-    F /*: Class<C> */,
-    elements /*: ElementDescriptor[] */,
-  ) {
-    var proto = F.prototype;
-
-    ["method", "field"].forEach(function(kind) {
-      elements.forEach(function(element /*: ElementDescriptor */) {
-        var placement = element.placement;
-        if (
-          element.kind === kind &&
-          (placement === "static" || placement === "prototype")
-        ) {
-          var receiver = placement === "static" ? F : proto;
-          _defineClassElement(receiver, element);
-        }
-      });
-    });
-  }
-
-  // InitializeInstanceElements
-  function _initializeInstanceElements /*::<C>*/(
-    O /*: C */,
-    elements /*: ElementDescriptor[] */,
-  ) {
-    ["method", "field"].forEach(function(kind) {
-      elements.forEach(function(element /*: ElementDescriptor */) {
-        if (element.kind === kind && element.placement === "own") {
-          _defineClassElement(O, element);
-        }
-      });
-    });
-  }
-
-  // DefineClassElement
-  function _defineClassElement /*::<C>*/(
-    receiver /*: C | Class<C> */,
-    element /*: ElementDescriptor */,
-  ) {
-    var descriptor /*: PropertyDescriptor */ = element.descriptor;
-    if (element.kind === "field") {
-      var initializer = element.initializer;
-      descriptor = {
-        enumerable: descriptor.enumerable,
-        writable: descriptor.writable,
-        configurable: descriptor.configurable,
-        value: initializer === void 0 ? void 0 : initializer.call(receiver),
-      };
-    }
-    Object.defineProperty(receiver, element.key, descriptor);
-  }
-
-  /*::
-
-  type Placements = {
-    static: Key[],
-    prototype: Key[],
-    own: Key[],
-  };
-
-  */
-
-  // DecorateClass
-  function _decorateClass(
-    elements /*: ElementDescriptor[] */,
-    decorators /*: ClassDecorator[] */,
-  ) /*: ElementsFinishers */ {
-    var newElements /*: ElementDescriptor[] */ = [];
-    var finishers /*: ClassFinisher[] */ = [];
-    var placements /*: Placements */ = { static: [], prototype: [], own: [] };
-
-    elements.forEach(function(element /*: ElementDescriptor */) {
-      _addElementPlacement(element, placements);
-    });
-
-    elements.forEach(function(element /*: ElementDescriptor */) {
-      if (!_hasDecorators(element)) return newElements.push(element);
-
-      var elementFinishersExtras /*: ElementFinishersExtras */ = _decorateElement(
-        element,
-        placements,
-      );
-      newElements.push(elementFinishersExtras.element);
-      newElements.push.apply(newElements, elementFinishersExtras.extras);
-      finishers.push.apply(finishers, elementFinishersExtras.finishers);
-    });
-
-    if (!decorators) {
-      return { elements: newElements, finishers: finishers };
-    }
-
-    var result /*: ElementsFinishers */ = _decorateConstructor(
-      newElements,
-      decorators,
-    );
-    finishers.push.apply(finishers, result.finishers);
-    result.finishers = finishers;
-
-    return result;
-  }
-
-  // AddElementPlacement
-  function _addElementPlacement(
-    element /*: ElementDescriptor */,
-    placements /*: Placements */,
-    silent /*: boolean */,
-  ) {
-    var keys = placements[element.placement];
-    if (!silent && keys.indexOf(element.key) !== -1) {
-      throw new TypeError("Duplicated element (" + element.key + ")");
-    }
-    keys.push(element.key);
-  }
-
-  // DecorateElement
-  function _decorateElement(
-    element /*: ElementDescriptor */,
-    placements /*: Placements */,
-  ) /*: ElementFinishersExtras */ {
-    var extras /*: ElementDescriptor[] */ = [];
-    var finishers /*: ClassFinisher[] */ = [];
-
-    for (
-      var decorators = element.decorators, i = decorators.length - 1;
-      i >= 0;
-      i--
-    ) {
-      // (inlined) RemoveElementPlacement
-      var keys = placements[element.placement];
-      keys.splice(keys.indexOf(element.key), 1);
-
-      var elementObject /*: ElementObjectInput */ = _fromElementDescriptor(
-        element,
-      );
-      var elementFinisherExtras /*: ElementFinisherExtras */ = _toElementFinisherExtras(
-        (0, decorators[i])(elementObject) /*: ElementObjectOutput */ ||
-          elementObject,
-      );
-
-      element = elementFinisherExtras.element;
-      _addElementPlacement(element, placements);
-
-      if (elementFinisherExtras.finisher) {
-        finishers.push(elementFinisherExtras.finisher);
-      }
-
-      var newExtras /*: ElementDescriptor[] | void */ =
-        elementFinisherExtras.extras;
-      if (newExtras) {
-        for (var j = 0; j < newExtras.length; j++) {
-          _addElementPlacement(newExtras[j], placements);
-        }
-        extras.push.apply(extras, newExtras);
-      }
-    }
-
-    return { element: element, finishers: finishers, extras: extras };
-  }
-
-  // DecorateConstructor
-  function _decorateConstructor(
-    elements /*: ElementDescriptor[] */,
-    decorators /*: ClassDecorator[] */,
-  ) /*: ElementsFinishers */ {
-    var finishers /*: ClassFinisher[] */ = [];
-
-    for (var i = decorators.length - 1; i >= 0; i--) {
-      var obj /*: ClassObject */ = _fromClassDescriptor(elements);
-      var elementsAndFinisher /*: ElementsFinisher */ = _toClassDescriptor(
-        (0, decorators[i])(obj) /*: ClassObject */ || obj,
-      );
-
-      if (elementsAndFinisher.finisher !== undefined) {
-        finishers.push(elementsAndFinisher.finisher);
-      }
-
-      if (elementsAndFinisher.elements !== undefined) {
-        elements = elementsAndFinisher.elements;
-
-        for (var j = 0; j < elements.length - 1; j++) {
-          for (var k = j + 1; k < elements.length; k++) {
-            if (
-              elements[j].key === elements[k].key &&
-              elements[j].placement === elements[k].placement
-            ) {
-              throw new TypeError("Duplicated element (" + elements[j].key + ")");
-            }
-          }
-        }
-      }
-    }
-
-    return { elements: elements, finishers: finishers };
-  }
-
-  // FromElementDescriptor
-  function _fromElementDescriptor(
-    element /*: ElementDescriptor */,
-  ) /*: ElementObject */ {
-    var obj /*: ElementObject */ = {
-      kind: element.kind,
-      key: element.key,
-      placement: element.placement,
-      descriptor: element.descriptor,
-    };
-
-    var desc = {
-      value: "Descriptor",
-      configurable: true,
-    };
-    Object.defineProperty(obj, Symbol.toStringTag, desc);
-
-    if (element.kind === "field") obj.initializer = element.initializer;
-
-    return obj;
-  }
-
-  // ToElementDescriptors
-  function _toElementDescriptors(
-    elementObjects /*: ElementObject[] */,
-  ) /*: ElementDescriptor[] */ {
-    if (elementObjects === undefined) return;
-    return toArray(elementObjects).map(function(elementObject) {
-      var element = _toElementDescriptor(elementObject);
-      _disallowProperty(elementObject, "finisher", "An element descriptor");
-      _disallowProperty(elementObject, "extras", "An element descriptor");
-      return element;
-    });
-  }
-
-  // ToElementDescriptor
-  function _toElementDescriptor(
-    elementObject /*: ElementObject */,
-  ) /*: ElementDescriptor */ {
-    var kind = String(elementObject.kind);
-    if (kind !== "method" && kind !== "field") {
-      throw new TypeError(
-        'An element descriptor\\'s .kind property must be either "method" or' +
-          ' "field", but a decorator created an element descriptor with' +
-          ' .kind "' +
-          kind +
-          '"',
-      );
-    }
-
-    var key = toPropertyKey(elementObject.key);
-
-    var placement = String(elementObject.placement);
-    if (
-      placement !== "static" &&
-      placement !== "prototype" &&
-      placement !== "own"
-    ) {
-      throw new TypeError(
-        'An element descriptor\\'s .placement property must be one of "static",' +
-          ' "prototype" or "own", but a decorator created an element descriptor' +
-          ' with .placement "' +
-          placement +
-          '"',
-      );
-    }
-
-    var descriptor /*: PropertyDescriptor */ = elementObject.descriptor;
-
-    _disallowProperty(elementObject, "elements", "An element descriptor");
-
-    var element /*: ElementDescriptor */ = {
-      kind: kind,
-      key: key,
-      placement: placement,
-      descriptor: Object.assign({}, descriptor),
-    };
-
-    if (kind !== "field") {
-      _disallowProperty(elementObject, "initializer", "A method descriptor");
-    } else {
-      _disallowProperty(
-        descriptor,
-        "get",
-        "The property descriptor of a field descriptor",
-      );
-      _disallowProperty(
-        descriptor,
-        "set",
-        "The property descriptor of a field descriptor",
-      );
-      _disallowProperty(
-        descriptor,
-        "value",
-        "The property descriptor of a field descriptor",
-      );
-
-      element.initializer = elementObject.initializer;
-    }
-
-    return element;
-  }
-
-  function _toElementFinisherExtras(
-    elementObject /*: ElementObject */,
-  ) /*: ElementFinisherExtras */ {
-    var element /*: ElementDescriptor */ = _toElementDescriptor(elementObject);
-    var finisher /*: ClassFinisher */ = _optionalCallableProperty(
-      elementObject,
-      "finisher",
-    );
-    var extras /*: ElementDescriptors[] */ = _toElementDescriptors(
-      elementObject.extras,
-    );
-
-    return { element: element, finisher: finisher, extras: extras };
-  }
-
-  // FromClassDescriptor
-  function _fromClassDescriptor(
-    elements /*: ElementDescriptor[] */,
-  ) /*: ClassObject */ {
-    var obj = {
-      kind: "class",
-      elements: elements.map(_fromElementDescriptor),
-    };
-
-    var desc = { value: "Descriptor", configurable: true };
-    Object.defineProperty(obj, Symbol.toStringTag, desc);
-
-    return obj;
-  }
-
-  // ToClassDescriptor
-  function _toClassDescriptor(obj /*: ClassObject */) /*: ElementsFinisher */ {
-    var kind = String(obj.kind);
-    if (kind !== "class") {
-      throw new TypeError(
-        'A class descriptor\\'s .kind property must be "class", but a decorator' +
-          ' created a class descriptor with .kind "' +
-          kind +
-          '"',
-      );
-    }
-
-    _disallowProperty(obj, "key", "A class descriptor");
-    _disallowProperty(obj, "placement", "A class descriptor");
-    _disallowProperty(obj, "descriptor", "A class descriptor");
-    _disallowProperty(obj, "initializer", "A class descriptor");
-    _disallowProperty(obj, "extras", "A class descriptor");
-
-    var finisher = _optionalCallableProperty(obj, "finisher");
-    var elements = _toElementDescriptors(obj.elements);
-
-    return { elements: elements, finisher: finisher };
-  }
-
-  function _disallowProperty(obj, name, objectType) {
-    if (obj[name] !== undefined) {
-      throw new TypeError(objectType + " can't have a ." + name + " property.");
-    }
-  }
-
   function _optionalCallableProperty /*::<T>*/(
     obj /*: T */,
     name /*: $Keys<T> */,
@@ -1721,23 +1771,6 @@ helpers.decorate = helper("7.1.5")`
     return value;
   }
 
-  // RunClassFinishers
-  function _runClassFinishers(
-    constructor /*: Class<*> */,
-    finishers /*: ClassFinisher[] */,
-  ) /*: Class<*> */ {
-    for (var i = 0; i < finishers.length; i++) {
-      var newConstructor /*: ?Class<*> */ = (0, finishers[i])(constructor);
-      if (newConstructor !== undefined) {
-        // NOTE: This should check if IsConstructor(newConstructor) is false.
-        if (typeof newConstructor !== "function") {
-          throw new TypeError("Finishers must return a constructor.");
-        }
-        constructor = newConstructor;
-      }
-    }
-    return constructor;
-  }
 `;
 
 helpers.classPrivateMethodGet = helper("7.1.6")`