EventEmitter: Remove unneeded Array.prototype.slice call
EventEmitter: Remove unneeded Array.prototype.slice call

apply() requires an Array-like object as second parameter. Arguments
objects fulfill this requirement. We already depend on this elsewhere.

Also:
* Change context from hard-coded eventEmitter to 'this'.
  It is not once()'s responsibility to set the context. That is already
  handled by on(). Just use whatever context was set by on() and forward
  it to the handler.
* Add 'return' statement so that the wrapper is functionality
  equivalent to the original handler. While we don't currently use
  this, it should not reject the return value.

Change-Id: Icd9ca5a1dc4561a33fda13cb1d46f73bbba38681

wikimedia/oojs,iSCInc/oojs,wikimedia/oojs