Cleanup git.js library.
Merge pull request #571 from dart-lang/link-markers-annotations

Linking markers to annotations.
Merge pull request #592 from dart-lang/cleanup_git

Cleanup git.js library.
Added two mock methods for previous CL
Merge pull request #598 from dart-lang/fix-ace-annotation-tests

Fixing tests broken by #571
Merge branch 'master' of https://github.com/dart-lang/spark into link-markers-editor
reorganized code, using StreamSubscription.
Created currentFile instance variable and set it every time the file switches.  Fixed the severity translations. Was mistakenly converting everything to warning.