Can't load Kernel binary: Invalid kernel binary format version. No active package webdev
Delete the bin/cache directory in your Flutter installation directory, then run flutter doctor.
Open Flutter SDK folder, go to bin/cache and delete all the files + folders then run flutter doctor, and then flutter upgrade.
instead of webdev serve try to use flutter packages pub global run webdev serve and for getting and upgrading packages you can use flutter packages get and flutter packages upgrade respectively.