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](https://github.com/flutter/flutter/blob/cd2f3f5e78409027d3c9014172708e4dec7f2185/packages/flutter_tools/lib/src/runner/flutter_command.dart#L1231)).