Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/constructor_private_name_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.constructors_test;
 
 // Regression test for C1 bug.
 
+@MirrorsUsed(targets: "test.constructors_test")
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
 class Foo {
   Foo._private();
 }
 
 class _Foo {
   _Foo._private();
 }
 
 main() {
   ClassMirror fooMirror = reflectClass(Foo);
   Symbol constructorName =
       (fooMirror.declarations[#Foo._private] as MethodMirror).constructorName;
   fooMirror.newInstance(constructorName, []);
 
   ClassMirror _fooMirror = reflectClass(_Foo);
   constructorName =
       (_fooMirror.declarations[#_Foo._private] as MethodMirror).constructorName;
   _fooMirror.newInstance(constructorName, []);
 }