Add prefix constraints for deferred imports.

tests/language/deferred_constraints_test.dart

Index: tests/language/deferred_constraints_test.dart
diff --git a/tests/language/deferred_constraints_test.dart b/tests/language/deferred_constraints_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..4a7dbe3b94300473754b9abb3324ce5833d24fa5
--- /dev/null
+++ b/tests/language/deferred_constraints_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_lib2.dart"; /// without_prefix: compile-time error
+@lazy import "deferred_constraints_lib.dart" as lib;
+import "deferred_constraints_lib2.dart" as lib; /// duplicate_prefix: compile-time error
+
+const lazy = const DeferredLibrary('lib');
+
+void main() {}