Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/symbol_validation_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.
 
 library symbol_validation_test;
 
+@MirrorsUsed(targets: "symbol_validation_test")
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 validSymbol(String string) {
   Expect.equals(string,
                 MirrorSystem.getName(new Symbol(string)),
                 'Valid symbol "$string" should be invertable');
   Expect.equals(string,
                 MirrorSystem.getName(MirrorSystem.getSymbol(string)),
                 'Valid symbol "$string" should be invertable');
@@ skipping 111 matching lines @@
   builtInIdentifiers.expand((w) => [w, "$w=", "x.$w" , "$w.x", "x.$w.x",
                                     "$w=", "x.$w="])
                     .forEach(validSymbol);
 
   var privateSymbols = [
     '_', '_x', 'x._y', 'x._', 'x.y._', 'x._.y', '_true'
   ];
   privateSymbols.forEach(invalidSymbol);
   privateSymbols.forEach(validPrivateSymbol);   /// 01: ok
 }