Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/synthetic_accessor_properties_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 test.synthetic_accessor_properties;
 
+@MirrorsUsed(targets: "test.synthetic_accessor_properties")
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 import 'stringify.dart';
 
 class C {
   String instanceField;
   final num finalInstanceField = 2;
 
   static bool staticField;
@@ skipping 44 matching lines @@
          ' type = Class(s(bool) in s(dart.core), top-level))', pm);
 
   mm = cm.staticMembers[#finalStaticField];
   expect('Method(s(finalStaticField) in s(C), synthetic, static, getter)', mm);
   Expect.equals(reflectClass(int), mm.returnType);
   Expect.listEquals([], mm.parameters);
 
   mm = cm.staticMembers[const Symbol('finalStaticField=')];
   Expect.isNull(mm);
 }