Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/generics_substitution_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 test.generics_substitution;
 
+@MirrorsUsed(targets: "test.generics_substitution")
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 class SuperGeneric<R, S> {
   R r;
   s(S s) {}
 }
 
 class Generic<T> extends SuperGeneric<T, int> {
   T t() {}
@@ skipping 29 matching lines @@
 
   Expect.equals(#R, r(superGenericDecl));
   Expect.equals(#S, s(superGenericDecl));
 
   Expect.equals(#T, r(superOfTAndInt));
   Expect.equals(#int, s(superOfTAndInt));
 
   Expect.equals(#String, r(superOfStringAndInt));
   Expect.equals(#int, s(superOfStringAndInt));
 }