Added backdrop to open folder
Merge branch 'master' of https://github.com/dart-lang/spark into polymer-backdrop_consistency
Merge branch 'master' of https://github.com/dart-lang/spark into polymer-backdrop_consistency
Merge branch 'master' of https://github.com/dart-lang/spark into polymer-backdrop_consistency
Made backdrop properties static.  Turned openFolder into a future and setup backdrop to disappear one done (on success)
Changed then() to whenComplete().  Removed traces
Removed code block
completing completer even when no folder is selected (_selectFolder).
Added overrides for open file and new file (and removed catch-all catch).
Fixed getter
Encapsulated backdrop functionality as much as possible
Merge pull request #755 from dart-lang/polymer-backdrop_consistency

Added backdrop to open folder