Eliminate the need for dart_style dev_dependency (flutter#20)

Apply dartfmt straight from the SDK.

Author: cbracken

pubspec.yaml

@@ -9,5 +9,4 @@ version: 0.2.0
 dependencies:
   path: ^1.4.0
 dev_dependencies:
-  dart_style:
   test: ^0.12.10

tool/travis.sh

@@ -1,9 +1,8 @@
 #!/bin/bash
 
 # Make sure dartfmt is run on everything
-# This assumes you have dart_style as a dev_dependency
 echo "Checking dartfmt..."
-NEEDS_DARTFMT="$(find lib test tool -name "*.dart" | xargs pub run dart_style:format -n)"
+NEEDS_DARTFMT="$(dartfmt -n lib test tool)"
 if [[ ${NEEDS_DARTFMT} != "" ]]
 then
   echo "FAILED"