let fire'd event be cancelable

Currently, fire is non-cancelable CustomEvent, and return only the details.

In this case, the fire can only be used to inform some events, and cannot do other thing.
// However, we can use detail, but it's not part of dom event.

Compares to YUI's fire, which returns something similar to the original method of dispatchEvent.
So I think its necessary to return the value of dispatchEvent.