Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/metadata_scope_test.dart

 // Copyright (c) 2014, 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.metadata_scope;
 
+@MirrorsUsed(targets: "test.metadata_scope")
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 class Annotation {
   final contents;
   const Annotation(this.contents);
   toString() => "Annotation($contents)";
 }
 
 // Note there is no compile-time constant 'foo' in scope. In particular, A.foo
@@ skipping 37 matching lines @@
   checkMetadata(reflectClass(B), [const Annotation(B.foo)]);
   checkMetadata(reflectClass(B).declarations[#T], [const Annotation(B.foo)]);
   checkMetadata(reflectClass(B).declarations[#foo], [const Annotation(B.foo)]);
   checkMetadata(reflectClass(B).declarations[#bar], [const Annotation(B.foo)]);
   // The top-level function baz, not C.baz.
   checkMetadata(reflectClass(C), [const Annotation(baz)]);
   // C.baz, not the top-level function baz.
   checkMetadata(reflectClass(C).declarations[#T], [const Annotation(C.baz)]);
   checkMetadata(reflectClass(C).declarations[#baz], [const Annotation(C.baz)]);
 }