Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/variable_is_const_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.variable_is_const;
 
+@MirrorsUsed(targets: "test.variable_is_const")
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
 class Class {
   const /// 01: compile-time error
       int instanceWouldBeConst = 1;
   var instanceNonConst = 2;
 
   static const staticConst = 3;
   static var staticNonConst = 4;
 }
 
 const topLevelConst = 5;
 var topLevelNonConst = 6;
 
 main() {
   bool isConst(m, Symbol s) => (m.declarations[s] as VariableMirror).isConst;
 
   ClassMirror cm = reflectClass(Class);
   Expect.isFalse(isConst(cm, #instanceWouldBeConst));
   Expect.isFalse(isConst(cm, #instanceNonConst));
   Expect.isTrue(isConst(cm, #staticConst));
   Expect.isFalse(isConst(cm, #staticNonConst));
 
   LibraryMirror lm = cm.owner;
   Expect.isTrue(isConst(lm, #topLevelConst));
   Expect.isFalse(isConst(lm, #topLevelNonConst));
 }