Add prefix constraints for deferred imports.

tests/compiler/dart2js/deferred_not_in_main_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.
 
 // Test of the graph segmentation algorithm used by deferred loading
 // to determine which elements can be deferred and which libraries
 // much be included in the initial download (loaded eagerly).
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
@@ skipping 50 matching lines @@
 import 'lib1.dart' as lib1;
 
 void main() {
   lib1.foo1();
 }
 """,
   "lib1.dart":"""
 library lib1;
 
 import 'dart:async';
-@def import 'lib2.dart';
+@def import 'lib2.dart' as lib2;
 
 const def = const DeferredLibrary('lib2');
 
 void foo1() {
-  def.load().then((_) => foo2());
+  def.load().then((_) => lib2.foo2());
 }
 """,
   "lib2.dart":"""
 library lib2;
 
 void foo2() {}
 """,
 };