Change onListen bailouts to asserts (dart-lang/stream_transform#68)

These transformers had a check that should never be true because the
behavior of the `StreamController` should prevent `onListen` from being
called multiple times while there is at least one active listener.
7 files changed
tree: b214f037dce298c246b11fcd877b86f0ebce8863
  1. pkgs/