* add trailing commas on list/map/parameters
* add trailing commas on Invocation with nb of arg>1
* add commas for widget containing widgets
* add trailing commas if instantiation contains trailing comma
* revert bad change
* Upgrade everything except matcher.
* Roll matcher (and test)
* Adjust tests that depend on flutter:test directly to depend on a shim
* Require use of package:test shim and remove other references to package:test
This fixes a rendering problem in the AnimatedChildSwitcher where it would add a new "previous" child each time it rebuilt, and if you did it fast enough, all of them would disappear from the page.
It also expands the API for AnimatedChildSwitcher to allow you to specify your own transition and/or layout builder for the transition.
Fixes#16226
Also:
* Add three explicit sizing modes to Stack for non-positioned
children: loose, expand, and passthrough. (All three are used.)
* Fix a bug whereby layers would try to paint in the same frame as
they were removed from layout (but not detached).
* Fix a bug whereby Offstage wasn't properly marking the parent dirty
when changing its sizedByParent flag.
* Explicitly make Overlay expand non-positioned children.
* Explicitly have InputDecoration pass through the constraints from
its Row to its Stack children.
Use the same technique for updating compositing bits as layout and
painting. This avoids walking the entire rendering tree when all you
need to update is a small subtree.
