Make the dart:mojo.internal import configuration specific.

mojo/dart/packages/mojo/lib/src/internal.dart

Index: mojo/dart/packages/mojo/lib/src/internal.dart
diff --git a/mojo/dart/packages/mojo/lib/src/internal.dart b/mojo/dart/packages/mojo/lib/src/internal.dart
new file mode 100644
index 0000000000000000000000000000000000000000..3c6b9b736cd7c894881af136761ac6265767d17f
--- /dev/null
+++ b/mojo/dart/packages/mojo/lib/src/internal.dart
@@ -0,0 +1,10 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+library mojo_internal;
+
+// Import 'internal_contract.dart' by default, but use 'dart:mojo.internal' if
+// the embedder supports it.
+export 'internal_contract.dart'
+  if (dart.library.mojo.internal) 'dart:mojo.internal';