static aborted() {
    const p = Promise.reject(shaka.util.AbortableOperation.abortError());
    // Silence uncaught rejection errors, which may otherwise occur any place
    // we don't explicitly handle aborted operations.
    p.catch(() => {});
    return new shaka.util.AbortableOperation(p, () => Promise.resolve());
  }
@return {!shaka.util.AbortableOperation} An operation which has already
  failed with the error OPERATION_ABORTED.
@export