Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/metadata_nested_constructor_call_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.
 
 // Regression test for Issue 17141.
 
 library test.metadata_nested_constructor_call;
 
+@MirrorsUsed(targets: "test.metadata_nested_constructor_call")
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 class Box {
   final contents;
   const Box([this.contents]);
 }
 
 class MutableBox {
   var contents;
@@ skipping 55 matching lines @@
   reflectClass(E).metadata;
   reflectClass(F).metadata;
   reflectClass(G).metadata;
   reflectClass(H).metadata;
   reflectClass(I).metadata;
   reflectClass(J).metadata;
   reflectClass(K).metadata;
   reflectClass(L).metadata;
   reflectClass(M).metadata;
 }