Add util function to get sha for a fileEntry.
Add file status types.
Add implmentation of git index.

It maintains the index of file status in the git repo.
Add git status.
Don't write unchanged files on commit.
nits.
Add write index support.
push
Addressed comments.
nit.
Added separator test before implementing
Changed example
Setup temporary styling (red background) for separator.  Height override is not working.
Merge branch 'master' of https://github.com/dart-lang/spark into umop-polymer-menu_separator
fix new file/folder to work on selection
Merge branch 'master' into work
add an error dialog
fix spacing nit
address comments
Merge pull request #792 from dart-lang/keertip_newfile

New file and new folder from selection