Polymer UI menu: cleanups, refactorings, etc.
Added "change theme" and "change keys" to Polymer UI menu

Might be temporary, if can't make them work.
Fixed widgets and Polymer UI: menu selection is plumbed up to the UI now
Merge branch 'fix-polymer-ui-menu' of https://github.com/ussuri/spark into sergey_menus
Merge branch 'upstream/master'
Addressed comments
Merge branch 'upstream/master'
WIP: switching to MVC paradigm in Polymer UI

SparkPolymerUI receives a handle to SparkModel, which Spark and SparkPolymer implement. SparkPolymerUI uses the model to map various events in the UI to ActionManager calls, etc.

There is currently a race condition: Spark ctor wants to access various elements of the UI, and so the UI (SparkPolymerUI) gets created too early, before enteredView() fires for various components, leading to a crash in SparkSelector on a null _target.
Merge branch 'upstream/master'
Add assert
Fix unnoticed refactoring artifact
batch resource change events
Added notion of modal dialogs
Merge branch 'master' of https://github.com/dart-lang/spark into sergey_menus
Cleanup
cleanup
Temporary testing setup
apply grind 4.4
tweak grind error message
Workaround to get chrome.restoreEntry() to work