Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/unnamed_mixin_application_test.dart

 // Copyright (c) 2016, 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.
 
 /// Test that the forwarding constructors of unnamed mixin applications are 
 /// included for reflection.
 
+library lib;
+
+@MirrorsUsed(targets: "lib")
 import 'dart:mirrors';
 
 class S {
   S();
   S.anUnusedName();
 }
 class M {}
 class C extends S with M {
   C();
 }
 
 main() {
   // Use 'C#', 'S+M#' and 'S#' but not 'S#anUnusedName' nor 'S+M#anUnusedName'.
   new C();
   // Disable tree shaking making 'S+M#anUnusedName' live.
   reflectClass(C);
 }