Add Symbol class.

dart/sdk/lib/core/symbol.dart

Index: dart/sdk/lib/core/symbol.dart
diff --git a/dart/sdk/lib/core/symbol.dart b/dart/sdk/lib/core/symbol.dart
new file mode 100644
index 0000000000000000000000000000000000000000..dba8e098fdabe7951e2b5e4b07242c5b0f5ff547
--- /dev/null
+++ b/dart/sdk/lib/core/symbol.dart
@@ -0,0 +1,19 @@
+// 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 the empty string and is not a valid identifier
+   * optionally followed by [:'=':].
+   */
+  external const Symbol(String name);
+}