Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/class_mirror_location_test.dart

 // Copyright (c) 2015, 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.class_location;
 
+@MirrorsUsed(targets: "test.class_location")
 import "dart:mirrors";
 import "package:expect/expect.dart";
 
 part 'class_mirror_location_other.dart';
 
 class ClassInMainFile {}
   class SpaceIndentedInMainFile {}
         class TabIndentedInMainFile {}
 
 abstract class AbstractClass {}
@@ skipping 42 matching lines @@
   
   // Another part.
   expectLocation(reflectClass(ClassInOtherFile), otherSuffix, 7, 1);
   expectLocation(reflectClass(SpaceIndentedInOtherFile), otherSuffix, 9, 3);
   expectLocation(reflectClass(TabIndentedInOtherFile), otherSuffix, 11, 2);
 
   // Synthetic classes.
   Expect.isNull(reflectClass(MA).superclass.location);
   Expect.isNull((reflect(main) as ClosureMirror).type.location);
 }