Added file marker change event.
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.
Ensuring that we only respond to ADD and DELETE marker events for the time being
Changed switch to comment
Added new property to mock
Changed _markerSubscription to _markerSubscription
Merge pull request #599 from dart-lang/link-markers-editor

Linking markers / annotations to editor for real-time annotations.