Add method flush to output stream

runtime/bin/output_stream.dart

Index: runtime/bin/output_stream.dart
diff --git a/runtime/bin/output_stream.dart b/runtime/bin/output_stream.dart
index 78663cb6d34f5403136e1b8eceaea52d6c47655d..cfbf8e51daa28c61ebbb555c7434b1ae3ff1565e 100644
--- a/runtime/bin/output_stream.dart
+++ b/runtime/bin/output_stream.dart
@@ -51,6 +51,13 @@ interface OutputStream {
   bool writeString(String string, [Encoding encoding]);
 
   /**
+   * Flushes data from any internal buffers as soon as possible. Note
+   * that the actual meaning of calling [flush] will depend on the
+   * actual type of the underlying communication channel.
+   */
+  void flush();
+
+  /**
    * Indicate that all data has been written to the output
    * stream. When all data has been written to the communication
    * channel it will be closed.