Use the latest dev channel Dart (#850)

As suggested by dart-lang/sdk#38334 (comment)

Author: nex3

.travis.yml

@@ -64,9 +64,8 @@ jobs:
     env: DART_CHANNEL=dev
   - <<: *dart-tests
     os: windows
-    # File system watching is extra flaky on Dart 2.5.0 on Windows (see
-    # dart-lang/sdk#38334).
-    env: DART_VERSION=2.4.1
+    # Work around dart-lang/sdk#38334 until 2.6.0 stable lands
+    env: DART_CHANNEL=dev
   - <<: *dart-tests
     os: osx