dart.vm.product is not set to true in --release builds

Flutter seems to have landed on using dart.vm.product to indicate release mode.

https://api.flutter.dev/flutter/foundation/kReleaseMode-constant.html

How Flutter sets it:
https://github.com/flutter/flutter/blob/b7c500fc6bf04e44156722cd053020595d1fbd35/packages/flutter_tools/lib/src/compile.dart#L197

I was looking for something similar for "dart web" builds, and failed to find such. Would be nice if the Flutter way worked for `webdev build --release` or at least if some way worked. :)

Start with the webdev build --release entry point and compare its behavior with Flutter's linked packages/flutter_tools/lib/src/compile.dart implementation. Done means release web builds set dart.vm.product to true, or provide an equivalent documented way to detect release mode.

Written by the indexing model from the issue text.