* 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

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

Covers lib/ in package:flutter.

This patch adds tests for some code paths we weren't hitting before and removes
some dead code that couldn't be tested because it was unreachable.

This patch starts working through the missing dartdocs in widgets.dart.

Also, add some missing docs to http.dart and widgets.dart.

Instead of PointerInputEvent having a "type" field, we now have a
different class for each pointer type.

This has ripple effects throughout the system.

I also did code cleanup in affected files while I was there.