Update .travis.yml (dart-lang/io#57)

pkgs/io/.travis.yml

diff --git a/pkgs/io/.travis.yml b/pkgs/io/.travis.yml
index 85c6b59..b50cdbf 100644
--- a/pkgs/io/.travis.yml
+++ b/pkgs/io/.travis.yml
@@ -12,7 +12,8 @@
   include:
   # Only validate formatting using the dev release
   - dart: dev
-    dart_task: dartfmt
+    dart_task:
+      dartfmt: sdk
 
 # Only building master means that we don't run two builds for each pull request.
 branches: