Add prefix constraints for deferred imports.

tests/lib/async/deferred/deferred_api_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 that the basic API for deferred/lazy loading works.  This
 // test deliberately does not test that the deferred elements throw a
 // NoSuchMethodError before the deferred library is loaded.  This
 // makes it possible to pass this test without having implemented
 // deferred loading correctly.
 
 import "package:expect/expect.dart";
 import 'dart:async';
 
 @lazy
-import 'deferred_api_library.dart';
+import 'deferred_api_library.dart' as lib;
 
 const lazy = const DeferredLibrary('deferred_api_library');
 
 main() {
   print('unittest-suite-wait-for-done');
 
   int counter = 0;
   lazy.load().then((bool didLoad) {
     Expect.isTrue(didLoad);
     Expect.equals(1, ++counter);
-    Expect.equals(42, foo('b'));
+    Expect.equals(42, lib.foo('b'));
     print('lazy was loaded');
   });
   Expect.equals(0, counter);
   lazy.load().then((bool didLoad) {
     Expect.isFalse(didLoad);
     Expect.equals(2, ++counter);
-    Expect.equals(42, foo('b'));
+    Expect.equals(42, lib.foo('b'));
     print('lazy was loaded');
     print('unittest-suite-success');
   });
   Expect.equals(0, counter);
 }