Don't delay IOSink.close() with no subscription to the next instance.

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 802486551935911386fd5440008f7801ad0ae267..dcdaf1ab4262d1fe977433a23ba546c7e70251cd 100644
--- a/sdk/lib/io/io_sink.dart
+++ b/sdk/lib/io/io_sink.dart
@@ -134,19 +134,16 @@ class _IOSinkImpl implements IOSink {
     }
     _isBound = true;
     // Wait for any sync operations to complete.
-    return _closeController().then((_) {
+    Future targetAddStream() {
       return _target.addStream(stream)
           .whenComplete(() {
             _isBound = false;
           });
-    });
-  }
-
-  Future _closeController() {
-    if (_controllerInstance == null) return new Future.immediate(null);
+    }
+    if (_controllerInstance == null) return targetAddStream();
     var future = _controllerCompleter.future;
     _controllerInstance.close();
-    return future;
+    return future.then((_) => targetAddStream());
   }
 
   Future close() {