Add prefix constraints for deferred imports.

tests/language/deferred_duplicate_prefix3_negative_test.dart

Index: tests/language/deferred_duplicate_prefix3_negative_test.dart
diff --git a/tests/language/deferred_duplicate_prefix3_negative_test.dart b/tests/language/deferred_duplicate_prefix3_negative_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..db66724a2a5ad334891a6d9aa1a026435746656a
--- /dev/null
+++ b/tests/language/deferred_duplicate_prefix3_negative_test.dart
@@ -0,0 +1,13 @@
+// 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 "dart:async";
+
+@lazy import "deferred_constraints_lib.dart" as lib;
+@lazy2 import "deferred_constraints_lib2.dart" as lib;
+
+const lazy = const DeferredLibrary('lib');
+const lazy2 = const DeferredLibrary('lib2');
+
+void main() {}