* Adds `TestFlutterView` to wrap `FlutterView` and allow modification of properties for testing
* Updates `TestPlatformDispatcher` to wrap its views in `TestFlutterView`s when they are changed
* Adds `viewOf` to `WidgetController` for finding a `TestFlutterView` related to the results of a `Finder`
* Exposes `platformDispatcher` and `implicitView` from `WidgetController` (and `WidgetTester`)
  * This is so the related `viewOf` method, which uses the `Finder` API, will be in a consistent place with these methods and other `Finder` APIs

….physicalSize` to allow for the web implementation where `physicalGeometry` and `physicalSize` can be decoupled.