Implement Symbol correctly in dart2js.

dart/tests/lib/mirrors/get_symbol_name_test.dart

+// Copyright (c) 2013, 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.
+
+import 'dart:mirrors' show MirrorSystem;
+
+expect(expected, actual) {
+  if (expected != actual) {
+    throw 'Expected: "$expected", but got "$actual"';
+  }
+}
+
+main() {
+  expect('fisk', MirrorSystem.getName(const Symbol('fisk')));
+  expect('fisk', MirrorSystem.getName(new Symbol('fisk')));
+}