A frequent request from the last Flutter developer survey was for an easier method of testing light/dark mode changes. Currently, a user needs to manually change the theme settings or adjust phone settings to see the difference. Instead we should add a toggle from the CLI, and eventually devtools/Intellij/Vscode that allows developers to override the current setting.

This can be implemented in several parts:

1. Add a framework service extension which overrides the Binding value of Brightness, similar to how the platform toggle does it.
2. Add a CLI option to toggle the brightness (b?) and wire it up.
3. For each IDE, expose functionality (as appropriate) to toggle the brightness. This would be left up to the owners of devtools, Intellij, and VSCode