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
add prelaunch participant for dart
Merge branch 'master' into work
fix nits
address comments
Fixed most widget examples; splitter and toolbar regressed/unfinished
changed paricipant to return bool
Allow overriding onSubmit event in dialogs.
Make onSubmit method private and refactor.
refactor.
fix messaging
Merge pull request #2335 from keertip/launch

add a pre launch check for pub packages
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