remove redundant casts in expressions like (x as A) as B where B is a subtype of A

[mraleph]

Flutter contains the following pattern of super-calls:

class MultiChildRenderObjectElement extends RenderObjectElement {
  @override
  MultiChildRenderObjectWidget get widget => super.widget as MultiChildRenderObjectWidget;
}

abstract class RenderObjectElement extends Element {
  @override
  RenderObjectWidget get widget => super.widget as RenderObjectWidget;
}

abstract class Element extends DiagnosticableTree implements BuildContext {
  @override
  Widget get widget => _widget;
  Widget _widget;
}

If the chain fully inlined we get the following graph:

v139 <- LoadField(v2 . _widget@246042623) T{Widget?}
AssertAssignable:14(v139 T{Widget?}, v125, ' in type cast', instantiator_type_args(v0), function_type_args(v0)) T{RenderObjectWidget?}
AssertAssignable:14(v139 T{RenderObjectWidget?}, v71, ' in type cast', instantiator_type_args(v0), function_type_args(v0))

Here the second assert assignable can "consume" the first one.

(@rakudrama brought my attention to these chains in dart-lang/language#1189)