* 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

It was causing problems rolling Flutter into Fuchsia

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

* Expose Gradient.radial

* cleanup doc

* update tests, null check

* add asset images

This PR adds a SweepGradient class, extending Gradient to expose the engine's ui.Gradient.sweep shader.

Similar to LinearGradient and RadialGradient - SweepGradients can be used in a BoxDecoration or passed to a Paint's shader.

Also adds test for interpolating gradient stops.

Make regular Alignment values look more like enumProperties while
leaving more complex AlignmentDirectional values unchanged.

This still is very limited in what it can lerp, but it sets the stage for arbitrary lerps later.

Unlike FractionalOffset, Alignment uses the center as the zero of the
coordinate system, which makes the RTL math work out much cleaner.

Also, make FractionalOffset into a subclass of Alignment so that clients
can continue to use FractionalOffset.

This reverts commit 4a04de5e.

* Revert "Pin args to 0.13.7 to avoid version conflict (#12208)"

This reverts commit 63802b3d.

* Revert "LinearGradient and RadialGradient RTL (#12204)"

This reverts commit daedbc84.