Add prefix constraints for deferred imports.

tests/compiler/dart2js_extra/deferred/deferred_constant4_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.
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 
 import 'dart:async';
 
-@lazy import 'deferred_class_library2.dart';
+@lazy import 'deferred_class_library2.dart' as lib;
 
 const lazy = const DeferredLibrary('deferred_class_library2');
 
 main() {
   asyncStart();
   lazy.load().then((bool didLoad) {
     Expect.isTrue(didLoad);
     // Only Gee2.n888 to make sure no other constant pulls in its super.
-    Expect.equals(888, new Gee2.n888().value);
+    Expect.equals(888, new lib.Gee2.n888().value);
     asyncEnd();
   });
 }