Internal libraries supported.

tests/language/internal_library_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 that a private library cannot be accessed from outside the platform.
+
+library internal_library_test;
+
+import 'dart:core'; // This loads 'dart:_foreign_helper' and 'patch:core'.
+import 'dart:_foreign_helper'; /// 01: compile-time error
+
+part 'dart:_foreign_helper'; /// 02: compile-time error
+
+void main() {
+  JS('int', '0'); /// 01: continued
+  JS('int', '0'); /// 02: continued
+}