Refresh folders when the focus is back on the app window
Added file deletion feature to the mock file system. Added a unit test for Workspace.checkForChange()
Added TODOs and improved comments. checkForChange() -> refresh().
Merge pull request #458 from dinhviethoa/302-refresh-folders

Refresh workspace when the focus is back on the app window.
workspace changes to support sync files
addressed comments
Merge pull request #466 from dart-lang/ws

workspace changes to support sync files
Merge branch 'upstream/master'