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-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;
+}();