Add method flush to output stream

runtime/bin/socket_stream_impl.dart

Index: runtime/bin/socket_stream_impl.dart
diff --git a/runtime/bin/socket_stream_impl.dart b/runtime/bin/socket_stream_impl.dart
index 101bf489854d8f44e7d4508584ae463db937e63c..1781ac27d6c5911ba908f30517a6b0a1d39a3a92 100644
--- a/runtime/bin/socket_stream_impl.dart
+++ b/runtime/bin/socket_stream_impl.dart
@@ -103,6 +103,10 @@ class _SocketOutputStream
         buffer, offset, (len == null) ? buffer.length - offset : len, true);
   }
 
+  void flush() {
+    // Nothing to do on a socket output stream.
+  }
+
   void close() {
     if (!_pendingWrites.isEmpty()) {
       // Mark the socket for close when all data is written.