Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/relation_assignable_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.relation_assignable;
 
+@MirrorsUsed(targets: "test.relation_assignable")
 import "dart:mirrors";
 
 import "package:expect/expect.dart";
 
 class Superclass {}
 class Subclass1 extends Superclass {}
 class Subclass2 extends Superclass {}
 
 typedef bool NumberPredicate(num x);
 typedef bool IntegerPredicate(int x);
@@ skipping 299 matching lines @@
   Expect.isFalse(Void.isAssignableTo(TFromB));
   Expect.isFalse(Void.isAssignableTo(TFromC));
 
   Expect.isTrue(Dynamic.isAssignableTo(Void));
   Expect.isTrue(Void.isAssignableTo(Dynamic));
 }
 
 main() {
   test(currentMirrorSystem());
 }