Method names should not be bound to body

As an artificat of compiling methods to named function expressions the
function name is being considered a "local" binding in the function
body. This means that we will throw errors anytime someone would want to
create a new local binding with the same name.

This is solved by assigning a symbol to function Identifiers that
indicates that they should not be considered local bindings.

packages/babel-helper-define-map/src/index.js

@@ -63,6 +63,11 @@ export function push(mutatorMap: Object, node: Object, kind: string, file, scope
   // infer function name
   if (scope && t.isStringLiteral(key) && (kind === "value" || kind === "initializer") && t.isFunctionExpression(value)) {
     value = nameFunction({ id: key, node: value, scope });
+
+    // Class methods don't have their name bound in the funciton body.
+    if (t.isClassMethod(node)) {
+      value.id[t.NOT_LOCAL_BINDING] = true
+    }
   }
 
   if (value) {

packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T6712/actual.js

@@ -0,0 +1,5 @@
+class A {
+  foo() {
+    const foo = 2;
+  }
+}

packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T6712/expected.js

@@ -0,0 +1,15 @@
+"use strict";
+
+var A = function () {
+  function A() {
+    babelHelpers.classCallCheck(this, A);
+  }
+
+  babelHelpers.createClass(A, [{
+    key: "foo",
+    value: function foo() {
+      var foo = 2;
+    }
+  }]);
+  return A;
+}();

packages/babel-traverse/src/scope/index.js

@@ -663,13 +663,17 @@ export default class Scope {
     // FunctionExpression - id
 
     if (path.isFunctionExpression() && path.has("id")) {
-      this.registerBinding("local", path.get("id"), path);
+      if (!path.get("id").node[t.NOT_LOCAL_BINDING]) {
+        this.registerBinding("local", path.get("id"), path);
+      }
     }
 
     // Class
 
     if (path.isClassExpression() && path.has("id")) {
-      this.registerBinding("local", path);
+      if (!path.get("id").node[t.NOT_LOCAL_BINDING]) {
+        this.registerBinding("local", path);
+      }
     }
 
     // Function - params, rest

packages/babel-types/src/constants.js

@@ -24,3 +24,4 @@ export const INHERIT_KEYS = {
 };
 
 export const BLOCK_SCOPED_SYMBOL = Symbol.for("var used to be block scoped");
+export const NOT_LOCAL_BINDING = Symbol.for("should not be considered a local binding");