feat: "--dart-define" option in test

[SuhwanCha]

flutter/flutter#34505

import 'package:flutter/foundation.dart';

String foo() {
  if (kReleaseMode) {
    return null;
  }
  return _calculateABunchOfStuffAndReturnALongString();
}

To cover source code above, --dart-define option is needed.

For example, to testing device in release mode,

flutter test --dart-define=dart.vm.product=true

[unicoderbot]

Great, I will work on a PR and push it soon.

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I've opened a pull request: #471