Relands https://github.com/flutter/flutter/pull/144752, which had to be reverted because the branch was stale. The original branch branched off `master` before https://github.com/flutter/flutter/pull/144734 landed. That PR introduced a new `AssetTransformer` call site.
This PR branch is identical to the original but with a new commit that addresses the new call site, [update new call sites](6bb5296a61).