L20 makes it impossible to use the class transformer's loose specified by presets passed to rollup-plugin-babel. This is because the class transform is manually added to the plugins list, which takes precedence over preset-provided plugins.

Is this intentional? If not, what about something like this?

const classCheck = transform( 'class Foo {}', options ).code;
if (~classCheck.indexOf('class Foo')) {
    options.plugins.push( classes );
}