a344fb1e2c
Updates the prepare_package.dart script to use the process_runner package (which is where the ProcessRunner implementation was moved to).
This package contains the custom build script used to compile Flutter for Web applications before support for JavaScript was added into the frontend_server. It is still used for test compilation today and is hosted here to reduce the number of dependencies that the flutter_tool has. For more information on the plans for flutter test --platform=chrome, see https://github.com/flutter/flutter/issues/50594