Removed differently typed getValue and setValue and turned into getter / setter with onUpdate stream.  Refactored accordingly.
Update (now updated) gets fired on finish of update and added onChanged gets fired immediately when a value is changed (for data binding).
Merge branch 'umop-preferences_phase1' of https://github.com/dart-lang/chromedeveditor into umop-preferences-json_cached
Merge branch 'umop-preferences-json_events' of https://github.com/dart-lang/chromedeveditor into umop-preferences-json_events
Merge branch 'master' of https://github.com/dart-lang/chromedeveditor into umop-preferences-json_events
Review fixes
Merge pull request #3272 from dart-lang/umop-preferences-json_events

JSON events (update and changed) in preferences