Add empty dart:platform library.

runtime/lib/platform_sources.gypi

Index: runtime/lib/platform_sources.gypi
diff --git a/runtime/lib/convert_sources.gypi b/runtime/lib/platform_sources.gypi
similarity index 72%
copy from runtime/lib/convert_sources.gypi
copy to runtime/lib/platform_sources.gypi
index 0f3b46b87b20a4a8fe86db748dd3fc873c3d20c6..dd1b34f6053d75ec28a9c7654330157e35b867c0 100644
--- a/runtime/lib/convert_sources.gypi
+++ b/runtime/lib/platform_sources.gypi
@@ -2,8 +2,9 @@
 # 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': [
-    'convert_patch.dart',
+    'platform_patch.dart',
   ],
 }