This rake task simply invokes mysql command with extra options from your config/database.yml
mysql --execute "SOURCE db/structure.sql;" --database db_name --user db_user --port db_port
Since heroku does not have mysql binaries on the dynos, you could add a buildpack for that https://elements.heroku.com/search/buildpacks?q=mysql
But, actually you don’t need it, just do local mysqldump, connect to your ClearDB database and import sql dump there.