Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/regress_26187_test.dart

 // Copyright (c) 2016, 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.
 
+@MirrorsUsed(targets: const ["A", "B", "f", "g"])
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 class A {
   const A();
 }
 
 class B {
   const B();
 }
 
 typedef void f(@A() int, String);
 
 typedef void g(@B() int, String);
 
 main() {
   ParameterMirror fParamMirror = (reflectType(f) as TypedefMirror).referent.parameters[0];
   ParameterMirror gParamMirror = (reflectType(g) as TypedefMirror).referent.parameters[0];
   Expect.equals('.A', MirrorSystem.getName(fParamMirror.metadata[0].type.qualifiedName));
   Expect.equals('.B', MirrorSystem.getName(gParamMirror.metadata[0].type.qualifiedName));
 }