Update dart:mirrors to use Symbol.

dart/tests/corelib/symbol_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.
 
 // Basic test of Symbol class.
 
 main() {
   var x;
   print(x = const Symbol('fisk'));
 
@@ skipping 65 matching lines @@
     throw 'unrelated objects are equal';
   }
 
   x.hashCode as int;
 
   new Symbol('fisk').hashCode as int;
 
   if (new Symbol('fisk').hashCode != x.hashCode) {
     throw "non-const Symbol's hashCode not equal to its const equivalent";
   }
+
+  if (new Symbol('') != const Symbol('')) {
+    throw 'empty Symbol not equals to itself';
+  }
 }