Move file_util out of git.js
move underscore-min.js and objects module out.

Move out third-party library underscore-min.js : Dart has native support
of underscore.

Move out objects module.
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
Merge pull request #50 from dart-lang/move_underscore.js

move underscore-min.js and objects module out.
Merge branch 'master' into zlib_dart
fix comments.
fix build failure.
Merge pull request #55 from dart-lang/fix_build

fix build failure.
merge to master.