perf(DOMSource): improve speed of isolated events()
perf(DOMSource): improve speed of isolated events()

Simulate a dropRepeats/filter operator using a mutable variable in the
scope, to avoid passing rootElement that are redundant. This avoids
redundantly re-attaching event listeners. Instead, event listeners are
attached only to *new* root elements.

maskinoshita/cyclejs,ntilwalli/cyclejs,feliciousx-open-source/cyclejs,staltz/cycle,usm4n/cyclejs,usm4n/cyclejs,feliciousx-open-source/cyclejs,cyclejs/cyclejs,maskinoshita/cyclejs,cyclejs/cyclejs,staltz/cycle,maskinoshita/cyclejs,ntilwalli/cyclejs,cyclejs/cycle-core,usm4n/cyclejs,usm4n/cyclejs,feliciousx-open-source/cyclejs,ntilwalli/cyclejs,feliciousx-open-source/cyclejs,cyclejs/cyclejs,cyclejs/cycle-core,maskinoshita/cyclejs,ntilwalli/cyclejs,cyclejs/cyclejs