Get the available anonymous $watch callback handler name

@inner
@param {Object} ctx the component instance
@return {string}
function getAnonymousWatcherName(ctx) {
    let counter = ctx.__$watch_counter;
    if (!counter) {
        counter = ctx.__$watch_counter = 0;
    }

    let handler = createAnonymousWatcherName(counter);
    ctx.__$watch_counter = counter + 1;
    return handler;
}