Constructor name change in async StreamTransformer

The release, retain, and callback creation methods seem to be missing.

Just get no such method errors when using it, however the API still
indicates they are present. When I look at:

https://github.com/dart-lang/js-interop/blob/master/lib/js.dart

I see no such methods so they have been removed. The example works once
again.

The release method will be retained for backwards compatability but the
README and other docs will be updated.

Have added big note to myself for the future. I expect to be wrong about
this.

Many thanks to N.Thillai Arasu for alerting me to this issue.