Add IOSink:flush().

sdk/lib/io/io_sink.dart

Index: sdk/lib/io/io_sink.dart
diff --git a/sdk/lib/io/io_sink.dart b/sdk/lib/io/io_sink.dart
index 042a469fb67c770216b9d1c07def35d8e9e2e02d..45390d88375c6552ce068286a2ea0e0d5bd8e2c6 100644
--- a/sdk/lib/io/io_sink.dart
+++ b/sdk/lib/io/io_sink.dart
@@ -46,6 +46,17 @@ abstract class IOSink implements StreamSink<List<int>>, StringSink {
   Future addStream(Stream<List<int>> stream);
 
   /**
+   * Returns a [Future] that completes once all buffered data is accepted by the
+   * to underlying [StreamConsumer].
+   *
+   * It's an error to call this method, while an [addStream] is incomplete.
+   *
+   * NOTE: This is not necessarily the same as the data being flushed by the
+   * operating system.
+   */
+  Future flush();
+
+  /**
    * Close the target.
    */
   Future close();
@@ -99,6 +110,13 @@ class _StreamSinkImpl<T> implements StreamSink<T> {
     return future.then((_) => targetAddStream());
   }
 
+  Future flush() {
+    // Adding an empty stream-controller will return a future that will complete
+    // when all data is done.
+    var controller = new StreamController()..close();
+    return addStream(controller.stream).then((_) => this);
+  }
+
   Future close() {
     if (_isBound) {
       throw new StateError("StreamSink is bound to a stream");