Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/reflect_uninstantiated_class_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.
 
 // A simple test that ensure that reflection works on uninstantiated classes.
 
+library lib;
+
+@MirrorsUsed(targets: "lib")
 import "dart:mirrors";
 
 class Foo {
   int a;
 }
 
 main() {
   // Do NOT instantiate Foo.
   var m = reflectClass(Foo);
   var field = publicFields(m).single;
   if (MirrorSystem.getName(field.simpleName) != 'a') {
     throw 'Expected "a", but got "${MirrorSystem.getName(field.simpleName)}"';
   }
   print(field);
 }
 
 publicFields(ClassMirror mirror) =>
     mirror.declarations.values.where(
         (x) => x is VariableMirror && !(x.isPrivate || x.isStatic));