Add prefix constraints for deferred imports.

tests/compiler/dart2js/deferred_load_graph_segmentation2_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 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 41 matching lines @@
 // Both lib1 and lib2 import lib3 directly and
 // both use lib3.foo3.  Therefore a shared output unit for lib1 and lib2 should
 // be created.
 //
 // lib1 and lib2 also import lib4 deferred, but lib1 uses lib4.bar1 and lib2
 // uses lib4.bar2.  So two output units should be created for lib4, one for each
 // import.
 const Map MEMORY_SOURCE_FILES = const {"main.dart": """
 import "dart:async";
 
-@def import 'lib.dart' show f1;
+@def import 'lib.dart' as lib show f1;
 import 'lib.dart' show f2;
 
 const def = const DeferredLibrary("deferred");
 
 void main() {
   print(f2());
   def.load().then((_) {
-    print(f1());
+    print(lib.f1());
   });
 }
 """, "lib.dart": """
 int f1 () {
   return 1;
 }
 
 int f2 () {
   return 2;
 }
 """,};