Remove unused spark-selection example (it's not a UI element anyway)
Update spark-button example
Rename assets -> resources to pacify `pub serve`
Fix/expand spark-button example
Merge remote-tracking branch 'upstream/master' into widget-examples-2
Missed one attr
Fixed most widget examples; splitter and toolbar regressed/unfinished
Merge remote-tracking branch 'upstream/master' into widget-examples-2
Added <spark-dialog> example
Merge pull request #2318 from ussuri/widget-examples-2

Fix and expand widget examples