Use the SDK dartfmt - and only on dev branch

Author: kevmoo

.travis.yml

@@ -6,9 +6,15 @@ dart:
 dart_task:
   # Run the tests -- include the default-skipped presubmit tests
   - test: --run-skipped
-  - dartfmt
+  - dartfmt: sdk
   - dartanalyzer
 
+matrix:
+  exclude:
+    - dart: stable
+      dart_task:
+        dartfmt: sdk
+
 # Only building master means that we don't run two builds for each pull request.
 branches:
   only: [master]