(Not a big deal)
on vue/src/directives/on.js:

reset function should be :
reset: function () {
var el = this.iframeBind
? this.el.contentWindow
: this.el
this.handler && el.removeEventListener(this.arg, this.handler)
},

Added code is "this.handler && "
Reason: at init, call to removeEventListener() while this.handler is null
according to specs, calls must always have the handler
https://developer.mozilla.org/en-US/docs/Web/API/EventTarget.removeEventListener

This happens because in update() function " this.reset() " (line 4253) is called before this.handler (line 4256) is defined.