Add Symbol class.

dart/runtime/lib/symbol_patch.dart

Index: dart/runtime/lib/symbol_patch.dart
diff --git a/dart/runtime/lib/symbol_patch.dart b/dart/runtime/lib/symbol_patch.dart
new file mode 100644
index 0000000000000000000000000000000000000000..67713487371ad5a5843055b030a3f6b09f229c1e
--- /dev/null
+++ b/dart/runtime/lib/symbol_patch.dart
@@ -0,0 +1,35 @@
+// 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 final RegExp _validationPattern =
+      new RegExp(r'^[a-zA-Z$][a-zA-Z$0-9_]*=?');
+
+  static _validate(String name) {
+    if (name is! String) throw new ArgumentError('name must be a String');
+    if (name.isEmpty) return;
+    if (name.startsWith('_')) {
+      throw new ArgumentError('"$name" is a private identifier');
+    }
+    if (!_validationPattern.hasMatch(name)) {
+      throw new ArgumentError(
+	  '"$name" is not an identifier or an empty String');
+    }
+    return name;
+  }
+
+  /* patch */ bool operator ==(other) {
+    return other is Symbol && _name == other._name;
+  }
+
+  /* patch */ int get hashCode {
+    const arbitraryPrime = 664597;
+    return 0x1fffffff & (arbitraryPrime * _name.hashCode);
+  }
+}