add basic naive constant folding of assignment expressions

src/babel/transformation/transformers/index.js

@@ -1,4 +1,7 @@
 export default {
+  //- builtin-prepass
+  "minification.constantFolding":          require("./minification/constant-folding"),
+
   //- builtin-setup
   strict:                                  require("./other/strict"),
   _explode:                                require("./internal/explode"),
@@ -7,7 +10,6 @@ export default {
   "minification.removeDebugger":           require("./minification/remove-debugger"),
   "minification.removeConsole":            require("./minification/remove-console"),
   "utility.inlineEnvironmentVariables":    require("./utility/inline-environment-variables"),
-  "minification.constantFolding":          require("./minification/constant-folding"),
   "minification.deadCodeElimination":      require("./minification/dead-code-elimination"),
   _modules:                                require("./internal/modules"),
   "spec.functionName":                     require("./spec/function-name"),

src/babel/transformation/transformers/minification/constant-folding.js

@@ -2,7 +2,28 @@ import * as t from "../../../types";
 
 export var metadata = {
   optional: true,
-  group: "builtin-setup"
+  group: "builtin-prepass"
+};
+
+export function AssignmentExpression() {
+  var left = this.get("left");
+  if (!left.isIdentifier()) return;
+
+  var binding = this.scope.getBinding(left.node.name);
+  if (!binding) return;
+
+  var right = this.get("right");
+  var evaluated = right.evaluate();
+  //if (evaluated.confident) binding.setValue(evaluated.value);
+}
+
+export var Scopable = {
+  exit() {
+    for (var name in this.scope.bindings) {
+      var binding = this.scope.bindings[name];
+      binding.clearValue();
+    }
+  }
 };
 
 export var Expression = {

src/babel/traversal/path/evaluation.js

@@ -100,6 +100,11 @@ export function evaluate(): { confident: boolean; value: any } {
       }
     }
 
+    if (path.isReferencedIdentifier()) {
+      var binding = path.scope.getBinding(node.name);
+      if (binding && binding.hasValue) return binding.value;
+    }
+
     if ((path.isIdentifier() || path.isMemberExpression()) && path.isReferenced()) {
       path = path.resolve();
       if (path) {

src/babel/traversal/scope/binding.js

@@ -10,6 +10,26 @@ export default class Binding {
     this.scope = scope;
     this.path  = path;
     this.kind  = kind;
+
+    this.clearValue();
+  }
+
+  /**
+   * Description
+   */
+
+  setValue(value) {
+    this.hasValue = true;
+    this.value    = value;
+  }
+
+  /**
+   * Description
+   */
+
+  clearValue() {
+    this.hasValue = false;
+    this.value    = null;
   }
 
   /**