Cleanups needed for async step-out

runtime/lib/async_patch.dart

Index: runtime/lib/async_patch.dart
diff --git a/runtime/lib/async_patch.dart b/runtime/lib/async_patch.dart
index 13cc9b81f240dce2aa623232dac25b32d57b8b13..15837ebdbd1fd8d2bf4a9de5d028caaaffc6824c 100644
--- a/runtime/lib/async_patch.dart
+++ b/runtime/lib/async_patch.dart
@@ -242,6 +242,10 @@ class _StreamImpl<T> {
   Function _generator;
 }
 
+void _completeOnAsyncReturn(Object completer, [value]) {

[rmacnak, 2017/03/27 21:05:51]

You always call this with both arguments, so don't make [value] optional.

[Cutch, 2017/03/27 22:31:20]

Done.

+  completer.complete(value);
+}
+
 /// Returns a [StackTrace] object containing the synchronous prefix for this
 /// asynchronous method.
 Object _asyncStackTraceHelper(Function async_op)