Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/accessor_cache_overflow_test.dart

 // Copyright (c) 2014, 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.
 
 // This test runs invokes getField and setField enough times to get cached
 // closures generated and with enough different field names to trip the path
 // that flushes the closure cache.
 
 library test.hot_get_field;
 
+@MirrorsUsed(targets: "test.hot_get_field")
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 const int optimizationThreshold = 20;
 
 main() {
   var digits =
       ['0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F'];
   var symbols = new List();
   for (var high in digits) {
@@ skipping 262 matching lines @@
   var vF7;
   var vF8;
   var vF9;
   var vFA;
   var vFB;
   var vFC;
   var vFD;
   var vFE;
   var vFF;
 }