code: function propagateError(error) {
      flow.pendingRejections_ += 1;
      return flow.timer.setTimeout(function() {
        flow.pendingRejections_ -= 1;
        flow.abortFrame_(error);
      }, 0);
    }
docs: Propagates an unhandled rejection to the parent ControlFlow in a
  future turn of the JavaScript event loop.
  @param {*} error The error value to report.
  @return {number} The key for the registered timeout.