Resolves#128046.
Adds a services API that allows flutter app developers to write app code that determines `--flavor` the app was built with.
This is implemented by having the tool adding the value of `--flavor` to its list of dart environment declarations, which will be available to the app at run time. Specifically,`FLUTTER_APP_FLAVOR` is set. I chose this implementation for its simplicity. There is some precedent for this, but only for web ([example](cd2f3f5e78/packages/flutter_tools/lib/src/runner/flutter_command.dart (L1231))).
