Move menu etc. back to spark_polymer_ui

Problems:
1) File filter control doesn't expand to full width on focus
2) "No matching files" is always displayed
3) The editor view has `height=0`. Changing that to `height=600px` shows the view.
Merge remote-tracking branch 'upstream/master' into move-menu-back-to-spark_polymer_ui

Conflicts:
	ide/app/spark_polymer.html
2 tiny fixes/cleanups
Merge remote-tracking branch 'upstream/master' into move-menu-back-to-spark_polymer_ui
Fix: #editorView was invisible
Fix: "No matching files" was always visible
Fix introduced bug: file tree view was too tall and would cause the UI jump up sometimes
Restore + rework: file filter expands to right, run and menu buttons hide
No-op refactoring
Clearer naming + fix "No matching files" showing up after clearing file filter
Renamings
Do not defocus file filter on ESC
Hide Run/menu buttons only when text is entered into file filter, not when it just gets focused
Merge remote-tracking branch 'upstream/master' into move-menu-back-to-spark_polymer_ui
Merge remote-tracking branch 'upstream/master' into move-menu-back-to-spark_polymer_ui

Conflicts:
	ide/app/spark_polymer.css
	ide/app/spark_polymer.dart
	ide/app/spark_polymer.html
Add cancel class which allows to past cancelable object.
Add more git exception types.
Add cancel to git clone.
Pass cancel object to fetcher and parser.
Add cancel to pack parse