Move file_util out of git.js
improve failure message for grind.dart
Update README.md

added description about defining DART_SDK env variable
pegging the version of chrome_gen that we use
Merge pull request #49 from dart-lang/refactor_file_util

Move file_util out of git.js
Merge pull request #52 from dart-lang/gen-pubspec

pegging the version of chrome_gen that we use
Merge pull request #51 from dart-lang/script-message

improve failure message for grind.dart
Merge master branch