Add prefix constraints for deferred imports.

tests/compiler/dart2js_extra/deferred/deferred_constant4_test.dart

Index: tests/compiler/dart2js_extra/deferred/deferred_constant4_test.dart
diff --git a/tests/compiler/dart2js_extra/deferred/deferred_constant4_test.dart b/tests/compiler/dart2js_extra/deferred/deferred_constant4_test.dart
index 7d0cc91594b21941b4f066af25a01016c348d9d9..e22d11b09e217d412c2add2bb9c6409afa60f657 100644
--- a/tests/compiler/dart2js_extra/deferred/deferred_constant4_test.dart
+++ b/tests/compiler/dart2js_extra/deferred/deferred_constant4_test.dart
@@ -7,7 +7,7 @@ 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');
 
@@ -16,7 +16,7 @@ main() {
   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();
   });
 }