function on_duringInit(futureDispatcher, events, fn) {
        on.call(futureDispatcher, events, fn);
    }
Utility for attaching an event handler to an object that has not YET had makeEventDispatcher() called
on it, but will in the future. Once 'futureDispatcher' becomes a real event dispatcher, any handlers
attached here will be retained.

Useful with core modules that have circular dependencies (one module initially gets an empty copy of the
other, with no on() API present yet). Unlike other strategies like waiting for htmlReady(), this helper
guarantees you won't miss any future events, regardless of how soon the other module finishes init and
starts calling trigger().

@param {!Object} futureDispatcher
@param {string} events
@param {?function(!{type:string, target:!Object}, ...)} fn