Small indexOf Perf Boost
Small indexOf Perf Boost

@megawac / @jdalton noticed a [pref regression](http://jsperf.com/indexof-pr2105)
in #2105. This should fix it.

[jsperf](http://jsperf.com/indexof-pr2105/3)

dfurtado/underscore,platinumazure/underscore,snackshen/underscore,zabefloyd/underscore,premkumargithub/underscore,aziz-boudi4/underscore,mitar/underscore,mdboop/underscore,ligson/underscore,Tim-Zhang/underscore,Joanna913/underscore,prosenjit-itobuz/underscore,beenlee/underscore,natac13/underscore,oss6/underscore,sdwang/underscore,aziz-boudi4/underscore,tnga/underscore-es,jashkenas/underscore,imsingh/underscore,imsingh/underscore,ElseZhang/underscore,thiswildorchid/underscore,PUSEN/underscore,janocat/underscore,craigmichaelmartin/underscore,JoaquinSiabra/underscore,xiaoqiang730730/underscore,snackshen/underscore,tnga/underscore-es,ijidan/underscore,beenlee/underscore,zabefloyd/underscore,Forever8/underscore,veg/underscore,jridgewell/underscore,thisismygithub/underscore,colemakdvorak/underscore,JoaquinSiabra/underscore,liminghui/underscore,lqjack/underscore,jianganglu/underscore,ligson/underscore,yuhualingfeng/underscore,kuychaco/underscore,falqas/underscore,gougouGet/underscore,jordanwink201/underscore,colemakdvorak/underscore,youprofit/underscore,Forever8/underscore,wjb12/underscore,dkusch/underscore,thiswildorchid/underscore,keepgoingwm/underscore,vinodbonala/underscore,craigmichaelmartin/underscore,greyhwndz/underscore,janocat/underscore,ijidan/underscore,emiel187/underscore,cintiamh/underscore,Joanna913/underscore,WangRex/underscore,rlugojr/underscore,huzion/underscore,veg/underscore,lqjack/underscore,JonAbrams/underscore,kwangkim/underscore,xiaoqiang730730/underscore,strongconjunctions/underscore,zhangguangyong/underscore,nickkneafsey/underscore,wxmfront/underscore,debutt/underscore,ElseZhang/underscore,huangnan1/2016gitStudy,lucasfanning/underscore,liyandalmllml/underscore,jiankers/underscore,kwangkim/underscore,dxuehu/underscore,premkumargithub/underscore,Feiyang1/underscore,dxuehu/underscore,ryanwholey/underscore,lucasfanning/underscore,dfurtado/underscore,irr/underscore,jonot/underscore,irr/underscore,emiel187/underscore,lis186/underscore,mjhasbach/underscore,Gimcrack/underscore,SeriaWei/underscore,huangnan1/2016gitStudy,sdwang/underscore,jordanwink201/underscore,vincent1988/underscore,elancom/underscore,polandeme/underscore,cgvarela/underscore,singhsterabhi/underscore,debutt/underscore,SerenoShen/underscore,Yangani/underscore,roychen-7/underscore,nickkneafsey/underscore,lis186/underscore,jolthoff/underscore,jiankers/underscore,ereddate/underscore,bkillenit/underscore,huzion/underscore,cnwhy/underscore,rlugojr/underscore,Gimcrack/underscore,cgvarela/underscore,jdelbello/underscore,singhsterabhi/underscore,zhangguangyong/underscore,kairyan/underscore,dkusch/underscore,ryanwholey/underscore,Tim-Zhang/underscore,gougouGet/underscore,PengShi-sp/underscore,JonAbrams/underscore,youprofit/underscore,jolthoff/underscore,SeriaWei/underscore,ThiagoGarciaAlves/underscore,jpalomar/underscore,greyhwndz/underscore,wxmfront/underscore,jdelbello/underscore,SerenoShen/underscore,falqas/underscore,oss6/underscore,PUSEN/underscore,prosenjit-itobuz/underscore,bkillenit/underscore,akre54/underscore,cnwhy/underscore,kairyan/underscore,vinodbonala/underscore,cintiamh/underscore,keepgoingwm/underscore,elancom/underscore,PengShi-sp/underscore,liyandalmllml/underscore,strongconjunctions/underscore,akre54/underscore,platinumazure/underscore,vincent1988/underscore,jianganglu/underscore,yekg880809/underscore,roychen-7/underscore,yekg880809/underscore,kuychaco/underscore,Yangani/underscore,ThiagoGarciaAlves/underscore,liminghui/underscore,jpalomar/underscore,mjhasbach/underscore,jashkenas/underscore,mitar/underscore,jfarid27/underscore,jonot/underscore,documentcloud/underscore,yuhualingfeng/underscore,ereddate/underscore,natac13/underscore,jridgewell/underscore,mdboop/underscore,WangRex/underscore,Feiyang1/underscore,mairasala/underscore,thisismygithub/underscore,polandeme/underscore,mairasala/underscore,jfarid27/underscore,documentcloud/underscore,wjb12/underscore