Stop running --no-strong analyzer (dart-lang/stream_transform#53)

With Dart 2 Strong mode is the only analysis we care about. Also stop
running extra tests on firefox.

pkgs/stream_transform/.travis.yml

diff --git a/pkgs/stream_transform/.travis.yml b/pkgs/stream_transform/.travis.yml
index 70dd977..326d6cf 100644
--- a/pkgs/stream_transform/.travis.yml
+++ b/pkgs/stream_transform/.travis.yml
@@ -9,7 +9,5 @@
 dart_task:
   - test
   - test -p chrome
-  - test -p firefox
   - dartfmt
   - dartanalyzer
-  - dartanalyzer: --no-strong .