Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/top_level_accessors_test.dart

 // Copyright (c) 2012, 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.top_level_accessors_test;
 
+@MirrorsUsed(targets: "test.top_level_accessors_test")
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
 var field;
 
 get accessor => field;
 
 set accessor(value) {
   field = value;
   return 'fisk';
 }
 
 main() {
   LibraryMirror library = currentMirrorSystem()
       .findLibrary(#test.top_level_accessors_test);
   field = 42;
   Expect.equals(42, library.getField(#accessor).reflectee);
   Expect.equals(87, library.setField(#accessor, 87).reflectee);
   Expect.equals(87, field);
   Expect.equals(87, library.getField(#accessor).reflectee);
 }