Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/set_field_with_final_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.set_field_with_final;
 
+@MirrorsUsed(targets: "test.set_field_with_final")
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 class C {
   final instanceField = 1;
   get instanceGetter => 2;
   static final staticFinal = 3;
   static get staticGetter => 4;
 }
 
@@ skipping 12 matching lines @@
                 (e) => e is NoSuchMethodError);
   Expect.throws(() => cm.setField(#staticGetter, 10),
                 (e) => e is NoSuchMethodError);
 
   LibraryMirror lm = cm.owner;
   Expect.throws(() => lm.setField(#toplevelFinal, 11),
                 (e) => e is NoSuchMethodError);
   Expect.throws(() => lm.setField(#toplevelGetter, 12),
                 (e) => e is NoSuchMethodError);
 }