Make transformer type parameters more concrete to let users control types.
Make transformer type parameters more concrete to let users control types.

google/streamy-dart,google/streamy-dart,ronlobo/streamy-dart,ronlobo/streamy-dart,srawlins/streamy-dart,ronlobo/streamy-dart,srawlins/streamy-dart,google/streamy-dart,srawlins/streamy-dart,google/streamy-dart,srawlins/streamy-dart,ronlobo/streamy-dart