Internal slot properties (#7947)

* Support internalSlots to babel-types and babel-generator

* Parsing support for internal slot properties

* Print internal slots in babel-generator

* Add whitespace before first internal slot property

packages/babel-generator/src/generators/flow.js

@@ -384,6 +384,7 @@ export function ObjectTypeAnnotation(node: Object) {
   const props = node.properties.concat(
     node.callProperties || [],
     node.indexers || [],
+    node.internalSlots || [],
   );
 
   if (props.length) {
@@ -413,6 +414,23 @@ export function ObjectTypeAnnotation(node: Object) {
   }
 }
 
+export function ObjectTypeInternalSlot(node: Object) {
+  if (node.static) {
+    this.word("static");
+    this.space();
+  }
+  this.token("[");
+  this.token("[");
+  this.print(node.id, node);
+  this.token("]");
+  this.token("]");
+  if (!node.method) {
+    this.token(":");
+    this.space();
+  }
+  this.print(node.value, node);
+}
+
 export function ObjectTypeCallProperty(node: Object) {
   if (node.static) {
     this.word("static");

packages/babel-generator/src/node/whitespace.js

@@ -209,6 +209,22 @@ nodes.ObjectTypeIndexer = function(node: Object, parent): ?WhitespaceObject {
   }
 };
 
+nodes.ObjectTypeInternalSlot = function(
+  node: Object,
+  parent,
+): ?WhitespaceObject {
+  if (
+    parent.internalSlots[0] === node &&
+    (!parent.properties || !parent.properties.length) &&
+    (!parent.callProperties || !parent.callProperties.length) &&
+    (!parent.indexers || !parent.indexers.length)
+  ) {
+    return {
+      before: true,
+    };
+  }
+};
+
 /**
  * Returns lists from node types that need whitespace.
  */