Add Symbol class.

dart/runtime/lib/symbol_patch.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.
+
+patch  class Symbol {
+  final String _name;
+  /* patch */ const Symbol(String name)
+    : this._name = _validate(name);
+
+  static _validate(String name) {
+    if (name is! String) throw new ArgumentError('name must be a String');
+    if (name.isEmpty) return;
+    if (name.startsWith('_')) {
+      throw new ArgumentError('"$name" is a private identifier');
+    }
+    if (!new RegExp(r'^[a-zA-Z$][a-zA-Z$0-9]*=?').hasMatch(name)) {
+      throw new ArgumentError(
+          '"$name" is not an identifier or the empty String');
+    }
+  }
+}