Add prefix constraints for deferred imports.

tests/language/deferred_constraints_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.
+
+import "dart:async";
+
+@lazy import "deferred_constraints_lib2.dart"; /// without_prefix: compile-time error
+import "deferred_constraints_lib2.dart" as lib; /// duplicate_prefix: compile-time error
+@lazy import "deferred_constraints_lib.dart" as lib;
+import "deferred_constraints_lib2.dart" as lib; /// duplicate_prefix2: compile-time error
+@lazy2 import "deferred_constraints_lib2.dart" as lib; /// duplicate_prefix3: compile-time error

[floitsch, 2014/02/18 12:19:25]

I would prefer new tests.

[sigurdm, 2014/02/18 13:31:47]

Ok, I have split the test.

+@lazy2 import "deferred_constraints_lib2.dart" as lib; /// duplicate_prefix3: ok
+
+const lazy = const DeferredLibrary('lib');
+const lazy2 = const DeferredLibrary('lib2'); /// duplicate_prefix3: continued
+
+void main() {}