Collection listen fix test

Not intentional, performance tweak on the splice, this property is pretty much for internal use, so that shouldn't be too much of a loss


> On Dec 1, 2014, at 16:48, Kris Kowal <notifications@github.com> wrote:
> 
> In ui/component.js:
> 
> > @@ -3405,7 +3405,7 @@ var RootComponent = Component.specialize( /** @lends RootComponent# */{
> >                      composer.needsFrame = false;
> >                      composer.frame(this._frameTime);
> >                  }
> > -                composerList.splice(0, composerListLength);
> > +                composerList.length = 0;
> This makes the length and content changes unobservable. Is that the intent?
> 
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