* 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.

I'm moving the assets in the assets-for-api-docs repo to a slightly different location to help with organization in that repo, so this PR points the doc URLs to the new location. The old assets won't be removed until this PR makes its way to the API docs website.

No documentation or code changes here, other than changing doc image URLs.

Also adds test for interpolating gradient stops.

This cleans up lerp, lerpFrom, lerpTo, and scale methods, and their
documentation.

Fixes https://github.com/flutter/flutter/issues/12377.

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

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.