As part of #32329, a Flutter app needs to be rendered inside the passed-in hostElement (if any).

Ensure that the following events are correctly handled:

• Routing
• Keyboard
• Mouse
• Locale
• Resize
• ??? others?

Also ensure that the Flutter web app doesn't modify the DOM outside of its root element (and attempt to not write to global variables).

Default behavior

If the hostElement is not passed, everything should behave as it does right now (taking over the whole page, etc...)

Docs

• See Preparing the DOM: initializeEngineUi section in the design doc