Add prefix constraints for deferred imports.

tests/compiler/dart2js_extra/deferred/deferred_unused_classes_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.
 
 // Test that deferred loader analyzer doesn't trip over unused classes.
 
 import "package:expect/expect.dart";
 import 'dart:async';
 
-@lazy import 'deferred_class_library.dart';
+@lazy import 'deferred_class_library.dart' as lib;
 
 const lazy = const DeferredLibrary('deferred_class_library');
 
 class Base {}
 
 class DerivedNotUsed extends Base {}
 
 main() {
 }