Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/regress_14304_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.
 
 // Regression test for Issue 14304.
 
+library lib;
+
+@MirrorsUsed(targets: "lib")
 import "dart:mirrors";
 import "package:expect/expect.dart";
 
 class A<T> {
   T m() {}
 }
 
 main() {
   ClassMirror a = reflectClass(A);
   TypeVariableMirror t = a.typeVariables[0];
   MethodMirror m = a.declarations[#m];
   
   Expect.equals(t, m.returnType);
 }