[dart fix] Allow changing base class and mixins

[goderbauer]

For flutter/flutter#123352 I would like to write a dart fix that changes all classes extending RootRenderObjectElement to instead extend RenderObjectElement and mix in RootElementMixin.

Basically, I would like to write a fix that changes

class Foo extends RootRenderObjectElement { ... }

to

class Foo extends RenderObjectElement with RootElementMixin { ... }

[goderbauer]

cc @bwilkerson

[bwilkerson]

Implementation note: I'd like to expand the capabilities of replacedBy to handle this case because it feels like we're replacing a class with another class and a mixin. I don't have a concrete proposal at this point for exactly how to do that.

[goderbauer]

This came up again, but this time in the other direction where we would want to fix

class Foo extends RenderObjectWidget with SlottedMultiChildRenderObjectWidgetMixin<S> {
  // ...
}

to

class Foo extends SlottedMultiChildRenderObjectWidget<S> {
  // ...
}