Use Dart 2.2 for tests

MichaelMarner · MichaelMarner · commit a79e46edb59b · 2019-07-09T08:34:03.000+09:30
Work around this issue: https://github.com/dart-lang/coverage/issues/249
diff --git a/.travis.yml b/.travis.yml
@@ -1,4 +1,6 @@
 language: dart
+dart:
+  - '2.2.0'
 install:
   - gem install coveralls-lcov
 script: ./tool/travis.sh
