dart_ui library Null safety

This file shims dart:ui in web-only scenarios, getting rid of the need to suppress analyzer warnings.

Classes

platformViewRegistry
Shim for web_ui engine.PlatformViewRegistry https://github.com/flutter/engine/blob/master/lib/web_ui/lib/ui.dart#L62
webOnlyAssetManager
Shim for web_ui engine.AssetManager. https://github.com/flutter/engine/blob/master/lib/web_ui/lib/src/engine/assets.dart#L12

Typedefs

VoidCallback = void Function()
Signature of callbacks that have no arguments and return no data.