Make StreamSubscription be the active part of a stream.

sdk/lib/io/mime_multipart_parser.dart

Index: sdk/lib/io/mime_multipart_parser.dart
diff --git a/sdk/lib/io/mime_multipart_parser.dart b/sdk/lib/io/mime_multipart_parser.dart
index 4bfbebe2a6151286345d8e3101f2f7dc6b263002..dc3f2069e8fb4377d685772cf0229ff1ae844973 100644
--- a/sdk/lib/io/mime_multipart_parser.dart
+++ b/sdk/lib/io/mime_multipart_parser.dart
@@ -101,11 +101,10 @@ class MimeMultipartTransformer
 
   Stream<MimeMultipart> bind(Stream<List<int>> stream) {
     _controller = new StreamController(
-        onPause: () {
-          _pauseStream();
-        },
-        onResume: () {
-          _resumeStream();
+        onPause: _pauseStream,
+        onResume:_resumeStream,
+        onCancel: () {
+          _subscription.cancel();
         },
         onListen: () {
           _subscription = stream.listen(
@@ -126,9 +125,6 @@ class MimeMultipartTransformer
               onError: (error) {
                 _controller.addError(error);
               });
-        },
-        onCancel: () {
-          _subscription.cancel();
         });
     return _controller.stream;
   }