auto merge of #3539 : jamesberlage/servo/fix/meaningful-is, r=jdm
auto merge of #3539 : jamesberlage/servo/fix/meaningful-is, r=jdm

This is intended to address #3488.  [AnyPrivate](http://doc.rust-lang.org/core/any/trait.AnyPrivate.html) is now in the Rust core, so we can leverage that instead of writing an implementation.

rixrix/servo,saneyuki/servo,canaltinova/servo,rixrix/servo,CJ8664/servo,froydnj/servo,thiagopnts/servo,karlito40/servo,srbhklkrn/SERVOENGINE,KiChjang/servo,akosel/servo,upsuper/servo,sadmansk/servo,szeged/servo,mt2d2/servo,echochamber/servo,DominoTree/servo,pyecs/servo,splav/servo,tempbottle/servo,splav/servo,splav/servo,paulrouget/servo,dagnir/servo,kindersung/servo,rentongzhang/servo,nick-thompson/servo,dati91/servo,youprofit/servo,akosel/servo,eddyb/servo,echochamber/servo,michaelwu/servo,Shraddha512/servo,notriddle/servo,youprofit/servo,wpgallih/servo,nnethercote/servo,srbhklkrn/SERVOENGINE,KiChjang/servo,nnethercote/servo,nnethercote/servo,g-k/servo,emilio/servo,WriterOfAlicrow/servo,avadacatavra/servo,upsuper/servo,GreenRecycleBin/servo,zentner-kyle/servo,sadmansk/servo,ryancanhelpyou/servo,snf/servo,boghison/servo,boghison/servo,kindersung/servo,shrenikgala/servo,srbhklkrn/SERVOENGINE,steveklabnik/servo,chotchki/servo,Shraddha512/servo,canaltinova/servo,mbrubeck/servo,karlito40/servo,kindersung/servo,mattnenterprise/servo,GreenRecycleBin/servo,aweinstock314/servo,Adenilson/prototype-viewing-distance,pyecs/servo,rixrix/servo,dsandeephegde/servo,dsandeephegde/servo,dvberkel/servo,jimberlage/servo,GreenRecycleBin/servo,fiji-flo/servo,boghison/servo,GreenRecycleBin/servo,szeged/servo,luniv/servo,mattnenterprise/servo,snf/servo,samfoo/servo,rnestler/servo,nnethercote/servo,larsbergstrom/servo,vks/servo,canaltinova/servo,akosel/servo,nnethercote/servo,dvberkel/servo,bjwbell/servo,evilpie/servo,RenaudParis/servo,runarberg/servo,mdibaiee/servo,paulrouget/servo,rixrix/servo,rixrix/servo,KiChjang/servo,pyfisch/servo,dmarcos/servo,dhananjay92/servo,larsbergstrom/servo,pgonda/servo,eddyb/servo,KiChjang/servo,splav/servo,RenaudParis/servo,rnestler/servo,juzer10/servo,notriddle/servo,Adenilson/prototype-viewing-distance,mbrubeck/servo,g-k/servo,wpgallih/servo,DominoTree/servo,fiji-flo/servo,aidanhs/servo,WriterOfAlicrow/servo,dagnir/servo,larsbergstrom/servo,thiagopnts/servo,dhananjay92/servo,kindersung/servo,nnethercote/servo,KiChjang/servo,snf/servo,RenaudParis/servo,Shraddha512/servo,jdramani/servo,rixrix/servo,anthgur/servo,jimberlage/servo,A-deLuna/servo,dsandeephegde/servo,sadmansk/servo,deokjinkim/servo,A-deLuna/servo,hyowon/servo,evilpie/servo,CJ8664/servo,kindersung/servo,codemac/servo,dvberkel/servo,sadmansk/servo,froydnj/servo,A-deLuna/servo,aweinstock314/servo,mukilan/servo,RenaudParis/servo,bfrohs/servo,ruud-v-a/servo,GreenRecycleBin/servo,dvberkel/servo,aidanhs/servo,larsbergstrom/servo,dagnir/servo,zhangjunlei26/servo,SimonSapin/servo,tempbottle/servo,nick-thompson/servo,wpgallih/servo,mdibaiee/servo,indykish/servo,caldwell/servo,canaltinova/servo,GyrosOfWar/servo,caldwell/servo,wpgallih/servo,huonw/servo,huonw/servo,nerith/servo,nerith/servo,zentner-kyle/servo,runarberg/servo,ConnorGBrewster/servo,dsandeephegde/servo,codemac/servo,deokjinkim/servo,hyowon/servo,GyrosOfWar/servo,tempbottle/servo,jimberlage/servo,ConnorGBrewster/servo,deokjinkim/servo,mt2d2/servo,aidanhs/servo,codemac/servo,jlegendary/servo,paulrouget/servo,karlito40/servo,tafia/servo,zhangjunlei26/servo,thiagopnts/servo,anthgur/servo,WriterOfAlicrow/servo,indykish/servo,splav/servo,tempbottle/servo,brendandahl/servo,walac/servo,hyowon/servo,mbrubeck/servo,Shraddha512/servo,kindersung/servo,AnthonyBroadCrawford/servo,pyecs/servo,ConnorGBrewster/servo,CJ8664/servo,youprofit/servo,peterjoel/servo,GreenRecycleBin/servo,ConnorGBrewster/servo,meh/servo,upsuper/servo,notriddle/servo,szeged/servo,pyecs/servo,j3parker/servo,mattnenterprise/servo,saratang/servo,rentongzhang/servo,boghison/servo,ruud-v-a/servo,michaelwu/servo,aidanhs/servo,sadmansk/servo,mukilan/servo,juzer10/servo,paulrouget/servo,ryancanhelpyou/servo,srbhklkrn/SERVOENGINE,jlegendary/servo,ruud-v-a/servo,DominoTree/servo,A-deLuna/servo,notriddle/servo,runarberg/servo,pyfisch/servo,rnestler/servo,huonw/servo,saneyuki/servo,jlegendary/servo,aidanhs/servo,peterjoel/servo,akosel/servo,jimberlage/servo,pyfisch/servo,boghison/servo,snf/servo,DominoTree/servo,rentongzhang/servo,ruud-v-a/servo,rnestler/servo,dagnir/servo,nrc/servo,mdibaiee/servo,boghison/servo,s142857/servo,zentner-kyle/servo,huonw/servo,zhangjunlei26/servo,cbrewster/servo,fiji-flo/servo,eddyb/servo,bjwbell/servo,upsuper/servo,shrenikgala/servo,pyfisch/servo,SimonSapin/servo,mbrubeck/servo,larsbergstrom/servo,shrenikgala/servo,kindersung/servo,evilpie/servo,jlegendary/servo,dati91/servo,ConnorGBrewster/servo,nick-thompson/servo,mt2d2/servo,runarberg/servo,echochamber/servo,vks/servo,wpgallih/servo,michaelwu/servo,AnthonyBroadCrawford/servo,zhangjunlei26/servo,jdramani/servo,nrc/servo,szeged/servo,dmarcos/servo,dati91/servo,Shraddha512/servo,avadacatavra/servo,dagnir/servo,dvberkel/servo,aweinstock314/servo,zhangjunlei26/servo,avadacatavra/servo,walac/servo,paulrouget/servo,saneyuki/servo,dhananjay92/servo,tschneidereit/servo,nerith/servo,wartman4404/servo,juzer10/servo,j3parker/servo,KiChjang/servo,vks/servo,dati91/se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