Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/private_class_field_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.
 
 // Test a private field name doesn't match the equivalent private name from
 // another library.
 
+library lib;
+
+@MirrorsUsed(targets: "lib")
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 import 'private_class_field_other.dart';
 
 void main() {
   var classMirror = reflectClass(C);
   // The symbol is private w/r/t the wrong library.
   Expect.throws(() => classMirror.getField(#_privateField),
                 (e) => e is NoSuchMethodError);
 
   Expect.equals(42, classMirror.getField(privateFieldSymbolInOther).reflectee);
 }