21932 Reland: Migration to PlatformDispatcher and multi-window (affects: desktop, cla: yes)

21933 Plumb through Dart entrypoint arguments on the Linux embedder (affects: desktop, cla: yes, needs tests, platform-linux)

22007 Fix FlTextInputPlugin tear down (affects: desktop, cla: yes, needs tests, platform-linux, waiting for tree to go green)

22094 Add plumbing for command line arguments on Windows (affects: desktop, cla: yes, needs tests, platform-windows, waiting for tree to go green)

22136 Fix warning when no entrypoint args provided. (affects: desktop, cla: yes, needs tests, platform-linux)

22268 Fix Linux handling of window exposure events (affects: desktop, cla: yes, needs tests, platform-linux)