Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/regress_19731_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.
 
 @metadata
 library regress_19731;
 
+@MirrorsUsed(targets: "regress_19731")
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 @metadata
 const metadata = const Object();
 
 class OneField {
   @metadata
   var onlyClassField;
 
   @metadata
   method() {}
 }
 
 @metadata
 method() {}
 
 main() {
   var classMirror = reflectType(OneField);
   var classFieldNames = classMirror.declarations.values
     .where((v) => v is VariableMirror).map((v) => v.simpleName).toList();
   Expect.setEquals([#onlyClassField], classFieldNames);
 
   var libraryMirror = classMirror.owner;
   var libraryFieldNames = libraryMirror.declarations.values
     .where((v) => v is VariableMirror).map((v) => v.simpleName).toList();
   Expect.setEquals([#metadata], libraryFieldNames);
 }