Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/reflectively_instantiate_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.
 
 // Ensure that otherwise uninstantiated classes can be instantiated
 // reflectively.
 
+library lib;
+
+@MirrorsUsed(targets: "lib")
 import "dart:mirrors";
 
 class Foo {
   int a;
 }
 
 main() {
   // Do NOT instantiate Foo.
   var m = reflectClass(Foo);
   var instance = m.newInstance(const Symbol(''), []);
   print(instance);
   bool threw = false;
   try {
     m.newInstance(#noSuchConstructor, []);
     throw 'Expected an exception';
   } on NoSuchMethodError catch (e) {
     print(e);
     threw = true;
   }
   if (!threw) throw 'Expected a NoSuchMethodError';
 }