This is a tracking bug for tracking the migration steps for implementing the multi-window API described in this design doc.

The steps for migration are:

• Convert engine internals to an API that understands multiple windows/screens, but only supports one one window.
• Convert the dart:ui Dart classes to use the internal engine API, but still only supports one window.
• Add platform detection of screens on the platform, reporting the proper size for the screens that the window appears on (still only managing one screen and one window)
• Add platform support for multiple screens and one window
• Add support for creating multiple windows, allowing creation and disposal, but not reconfiguration.
• Add support for re-configuring windows.

The initial PRs for the first step are flutter/engine#18368 and #57178