Most error messages in gradle_errors.dart assume a unix like path separator/environment

Examples
path separators 
https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/android/gradle_errors.dart#L114
https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/android/gradle_errors.dart#L149

`./gradlew` vs `./gradlew.bat`
https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/android/gradle_errors.dart#L524