Half-way through: dumb UI works, but underlying Dart is mostly disabled

...as a result, the toolbar and splitter show, but buttons don't do anything, and file treeview pane and edit pane are empty.
Fix the layout after a recent breaking change in polymer-flex-panel
Merge branch 'upstream/master'

Conflicts:
	ide/app/spark_polymer.css
	ide/app/spark_polymer.dart
	ide/app/spark_polymer.html
Midway through intaking recent changes in non-polymer UI

CSS done, everything works. Expecting more CSS changes necessary after syncing HTML.
Make polymer_elements a direct dependency (was indirect via spark_widgets)
3/4 of the way through intaking recent changes in non-polymer UI

HTML done, but widgets are still broken, to do next.
Merge branch 'upstream/master'

Conflicts:
	ide/app/spark_polymer.html
Fixed widgets (superclass Widget moved from src/ to common/)
Fixed path to background image
Fixed mistakenly quoted CSS property value
Fixed the toolbar + minor style fixes - the UI looks normal again
Fixed toolbar border
Avoid potential race condition and localizing code to be changed in SparkPolymer
Move showStatus() to a proper location.
Adjust non-Polymer UI to make it reusable in redesigned Polymer UI, and reuse it.

An additional bonus: previously spread out knowledge about the exact ids and structure of spark.html is now much more concentrated in Spark class. Other classes/functions get already extracted Elements from it to work on.

This is a bridge version just to allow continued reuse of Spark in SparkPolymer: it still binds to data and events dynamically, pulling Elements from the HTML and modifying them in Dart. After or just before the complete switch to Polymer UI, this will be redone further using static binding, with Dart preparing and binding observable data to Polymer elements' attributes, letting the elements do presenting themselves.
Almost final version of v.1.0 of redesigned Polymer UI

- Menu doesn't drop down when the button is clicked (but can be forced to display in DevTools)
- Menu doesn't look nice
- Dialogs don't pop-up when a respective menu item is clicked
- Item order in the menu is not as wanted

Many of these are not critical anyway, as the UI will be further redone in a more Polymer-y way:

- Prepare data and event handlers in the app and pass it to elements, which display it as appropriate.
- Dialogs, menus etc. will be completely replaced by respective Spark widgets.
Move getElement() from SparkPolymerUi to Widget
Merge branch 'upstream/master'

Conflicts:
	ide/pubspec.yaml
Merge branch 'master'

Conflicts:
	ide/app/spark.dart
Fixed merge