Add StreamChannelTransformer.

lib/src/isolate_channel.dart

Index: lib/src/isolate_channel.dart
diff --git a/lib/src/isolate_channel.dart b/lib/src/isolate_channel.dart
index c6645438d6b02daf99ff7d916df884cabc2d59e5..46375c986ece4bf296baa4719655a9fff9268252 100644
--- a/lib/src/isolate_channel.dart
+++ b/lib/src/isolate_channel.dart
@@ -128,7 +128,7 @@ class _SendPortSink<T> implements StreamSink<T> {
     if (_inAddStream) {
       throw new StateError("Cannot add stream while adding stream.");
     }
-    if (_isDone) return;
+    if (_isDone) return new Future.value();

[Bob Nystrom, 2016/01/27 19:30:40]

I wonder if it would be cleaner to make the function async? Then you could await
the completer's future below too.

[nweiz, 2016/01/27 23:23:49]

It seems weird to have the errors thrown asynchronously when the rest of the
methods throw them synchronously.

 
     _inAddStream = true;
     var completer = new Completer.sync();