Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/generic_superclass_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;
+
 import 'package:expect/expect.dart';
+
+@MirrorsUsed(targets: "lib")
 import 'dart:mirrors';
 
 
 class A<T> {}
 class B extends A<U>{}
 class C extends A<C> {}
 class D<T> extends A<T> {}
 class E<X,Y> extends G<H<Y>> {}
 class F<X> implements A<X> {}
 class FF<X,Y> implements G<H<Y>> {}
@@ skipping 95 matching lines @@
 void testObject() {
   ClassMirror object = reflectClass(Object);
   Expect.equals(null, object.superclass);
 }
 
 main() {
   testOriginals();
   testInstances();
   testObject();
 }