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
pubspec tweaks, in support of moving towards pub build
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.
fix a speed issue w/ the zlib encoder and js interop
Merge pull request #373 from dart-lang/zlib_speed

fix a speed issue w/ the zlib encoder and js interop
Add git clone.
Merge pull request #367 from dart-lang/pub_build

pubspec tweaks, in support of moving towards pub build
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.