Add method flush to output stream

runtime/bin/list_stream_impl.dart

Index: runtime/bin/list_stream_impl.dart
diff --git a/runtime/bin/list_stream_impl.dart b/runtime/bin/list_stream_impl.dart
index 82520e390084f78c1a122641e2327bfafb77399d..436983e9784475e609f801ccc322eb7919d271a2 100644
--- a/runtime/bin/list_stream_impl.dart
+++ b/runtime/bin/list_stream_impl.dart
@@ -64,6 +64,10 @@ class _ListOutputStream extends _BaseOutputStream implements ListOutputStream {
     return true;
   }
 
+  void flush() {
+    // Nothing to do on a list output stream.
+  }
+
   void close() {
     if (_streamMarkedClosed) throw new StreamException.streamClosed();
     _streamMarkedClosed = true;