Add empty dart:platform library.

sdk/lib/platform/platform_sources.gypi

Index: sdk/lib/platform/platform_sources.gypi
diff --git a/sdk/lib/isolate/isolate_sources.gypi b/sdk/lib/platform/platform_sources.gypi
similarity index 61%
copy from sdk/lib/isolate/isolate_sources.gypi
copy to sdk/lib/platform/platform_sources.gypi
index fdc0f958df04d6c024cdffe043647719ba3e4cd7..6bca2b3f4d65111dccc91af968754d9c97f34673 100644
--- a/sdk/lib/isolate/isolate_sources.gypi
+++ b/sdk/lib/platform/platform_sources.gypi
@@ -1,11 +1,11 @@
-# Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+# 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.
 
+# This file contains all sources for the dart:platform library.
 {
   'sources': [
-    'isolate.dart',
+    'platform.dart',
     # The above file needs to be first as it lists the parts below.
-    'isolate_stream.dart',
   ],
 }