Make toString visible on enum classes, as required by proposed spec.

tests/language/enum_mirror_test.dart

+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// SharedOptions=--enable-enum
+
+import 'dart:mirrors';
+
+import 'package:expect/expect.dart';
+
+enum Foo { BAR, BAZ }
+
+main() {
+  Expect.equals('Foo.BAR', Foo.BAR.toString());
+  var name = reflect(Foo.BAR).invoke(#toString, []).reflectee;
+  Expect.equals('Foo.BAR', name);
+}