Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/array_tracing_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 lib;
+
+@MirrorsUsed(targets: const ["lib", "dart.core"])
 import 'dart:mirrors';
 
 class A {
   static var field;
 }
 
 main() {
   MirrorSystem mirrors = currentMirrorSystem();
   ClassMirror a = reflectClass(A);
   var array = [42];
   A.field = array;
   var field = a.getField(#field);
   field.invoke(#clear, []);
   if (array.length == 1) throw 'Test failed';
 }