Add Symbol class.

dart/sdk/lib/core/symbol.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.
+
+part of dart.core;
+
+/// Opaque name used by mirrors, invocations and [Function.apply].
+class Symbol {
+
+  /**
+   * Constructs a new Symbol.
+   *
+   * An [ArgumentError] is thrown if [name] starts with an underscore,
+   * or if [name] is not a [String].  An [ArgumentError] is thrown if
+   * [name] is not an empty string and is not a valid identifier
+   * optionally followed by [:'=':].
+   */
+  external const Symbol(String name);
+
+  external bool operator ==(other);
+
+  external int get hashCode;
+}