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.
Fixes#59495
Adds flutter.ext.brightnessOverride service protocol which either queries the current platform brightness, or overrides it to a new value. This accepts either Brightness.light or Brightness.dark as a value.
Adds a CLI toggle b which allows the setting to be toggled manually.
Requires an update to the MediaQuery, to conditionally use a debug override when not in release mode
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