Backdrop on Import File and Import Folder.  Import Folder's does not stick
Fixed return on importFolder and cleaned up code.
Resolved conflicts
Merge branch 'umop-import-backdrop' of https://github.com/dart-lang/spark into umop-import-backdrop
Review fixes
Merge pull request #2065 from dart-lang/umop-import-backdrop

Backdrop for Import Folder and Import File dialogs.