Some changes were required to stream_transformer_test to make it strong
mode clean. Other minor changes involved migrating from unittest->test.
Bug:
Change-Id: Ia21144090710137e4803c651cf9e3fd62e17f5f7
Reviewed-on: https://dart-review.googlesource.com/3707
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>