Add istanbul for running node coverage reports.
Add istanbul for running node coverage reports.

`tools/test-js-with-node cover` needs istanbul to be installed in
order to work; we might as well install it by default rather than
having it be an extra step users need to deal with.

Of course, since this is only needed in the development environment,
this could suggest we want to fork/conditionalize package.json, but I
think for now it's reasonable to just install everything we use
somewhere -- the npm list is still pretty short and we have that issue
anyway with webpack-dev-server.

jphilipsen05/zulip,rishig/zulip,Galexrt/zulip,andersk/zulip,peguin40/zulip,kou/zulip,kou/zulip,showell/zulip,niftynei/zulip,aakash-cr7/zulip,Vallher/zulip,peguin40/zulip,tommyip/zulip,peiwei/zulip,synicalsyntax/zulip,Galexrt/zulip,zulip/zulip,Vallher/zulip,Jianchun1/zulip,verma-varsha/zulip,esander91/zulip,tommyip/zulip,SmartPeople/zulip,dhcrzf/zulip,brockwhittaker/zulip,vikas-parashar/zulip,showell/zulip,susansls/zulip,showell/zulip,SmartPeople/zulip,timabbott/zulip,TigorC/zulip,peiwei/zulip,tommyip/zulip,Galexrt/zulip,esander91/zulip,sup95/zulip,dhcrzf/zulip,SmartPeople/zulip,punchagan/zulip,punchagan/zulip,sup95/zulip,shubhamdhama/zulip,synicalsyntax/zulip,blaze225/zulip,dawran6/zulip,TigorC/zulip,Frouk/zulip,ahmadassaf/zulip,cosmicAsymmetry/zulip,cosmicAsymmetry/zulip,eeshangarg/zulip,ryansnowboarder/zulip,j831/zulip,AZtheAsian/zulip,PhilSk/zulip,ryanbackman/zulip,j831/zulip,christi3k/zulip,joyhchen/zulip,jackrzhang/zulip,dawran6/zulip,ryansnowboarder/zulip,krtkmj/zulip,Frouk/zulip,vikas-parashar/zulip,hackerkid/zulip,tommyip/zulip,shubhamdhama/zulip,ryansnowboarder/zulip,paxapy/zulip,vaidap/zulip,karamcnair/zulip,SmartPeople/zulip,souravbadami/zulip,samatdav/zulip,aakash-cr7/zulip,mohsenSy/zulip,ahmadassaf/zulip,souravbadami/zulip,Vallher/zulip,zulip/zulip,isht3/zulip,hackerkid/zulip,punchagan/zulip,dhcrzf/zulip,Frouk/zulip,TigorC/zulip,sharmaeklavya2/zulip,alliejones/zulip,ahmadassaf/zulip,brainwane/zulip,reyha/zulip,alliejones/zulip,dwrpayne/zulip,calvinleenyc/zulip,brockwhittaker/zulip,brainwane/zulip,arpith/zulip,brainwane/zulip,Frouk/zulip,esander91/zulip,dhcrzf/zulip,dwrpayne/zulip,umkay/zulip,jainayush975/zulip,alliejones/zulip,dwrpayne/zulip,dwrpayne/zulip,hackerkid/zulip,kou/zulip,brainwane/zulip,sup95/zulip,grave-w-grave/zulip,peguin40/zulip,grave-w-grave/zulip,jackrzhang/zulip,rishig/zulip,showell/zulip,Vallher/zulip,susansls/zulip,andersk/zulip,vaidap/zulip,Frouk/zulip,ryansnowboarder/zulip,zulip/zulip,amanharitsh123/zulip,blaze225/zulip,peiwei/zulip,KingxBanana/zulip,jackrzhang/zulip,jackrzhang/zulip,dwrpayne/zulip,cosmicAsymmetry/zulip,hackerkid/zulip,blaze225/zulip,JPJPJPOPOP/zulip,esander91/zulip,j831/zulip,dhcrzf/zulip,punchagan/zulip,christi3k/zulip,blaze225/zulip,esander91/zulip,vikas-parashar/zulip,mohsenSy/zulip,PhilSk/zulip,krtkmj/zulip,peiwei/zulip,niftynei/zulip,joyhchen/zulip,verma-varsha/zulip,verma-varsha/zulip,esander91/zulip,KingxBanana/zulip,mahim97/zulip,JPJPJPOPOP/zulip,ryansnowboarder/zulip,sharmaeklavya2/zulip,brockwhittaker/zulip,rht/zulip,PhilSk/zulip,isht3/zulip,brainwane/zulip,zacps/zulip,isht3/zulip,shubhamdhama/zulip,ryansnowboarder/zulip,krtkmj/zulip,souravbadami/zulip,umkay/zulip,ahmadassaf/zulip,sup95/zulip,amyliu345/zulip,AZtheAsian/zulip,calvinleenyc/zulip,verma-varsha/zulip,mahim97/zulip,arpith/zulip,ryanbackman/zulip,zacps/zulip,andersk/zulip,vikas-parashar/zulip,dhcrzf/zulip,shubhamdhama/zulip,dwrpayne/zulip,zulip/zulip,christi3k/zulip,jrowan/zulip,amanharitsh123/zulip,kou/zulip,TigorC/zulip,amanharitsh123/zulip,mahim97/zulip,peiwei/zulip,zacps/zulip,ahmadassaf/zulip,vaidap/zulip,sonali0901/zulip,rht/zulip,eeshangarg/zulip,Jianchun1/zulip,dawran6/zulip,shubhamdhama/zulip,mahim97/zulip,joyhchen/zulip,peiwei/zulip,aakash-cr7/zulip,umkay/zulip,vaidap/zulip,joyhchen/zulip,blaze225/zulip,amanharitsh123/zulip,vikas-parashar/zulip,punchagan/zulip,eeshangarg/zulip,calvinleenyc/zulip,Juanvulcano/zulip,dattatreya303/zulip,samatdav/zulip,mohsenSy/zulip,jrowan/zulip,Galexrt/zulip,amanharitsh123/zulip,vabs22/zulip,peguin40/zulip,mohsenSy/zulip,Vallher/zulip,peguin40/zulip,sharmaeklavya2/zulip,Jianchun1/zulip,jphilipsen05/zulip,KingxBanana/zulip,dwrpayne/zulip,Vallher/zulip,ahmadassaf/zulip,dattatreya303/zulip,jackrzhang/zulip,Juanvulcano/zulip,sharmaeklavya2/zulip,calvinleenyc/zulip,jphilipsen05/zulip,esander91/zulip,sup95/zulip,AZtheAsian/zulip,jrowan/zulip,arpith/zulip,Juanvulcano/zulip,paxapy/zulip,SmartPeople/zulip,alliejones/zulip,reyha/zulip,Juanvulcano/zulip,vabs22/zulip,niftynei/zulip,rishig/zulip,arpith/zulip,sonali0901/zulip,Frouk/zulip,zacps/zulip,eeshangarg/zulip,susansls/zulip,SmartPeople/zulip,AZtheAsian/zulip,synicalsyntax/zulip,tommyip/zulip,dattatreya303/zulip,ryanbackman/zulip,krtkmj/zulip,reyha/zulip,brockwhittaker/zulip,christi3k/zulip,vabs22/zulip,amyliu345/zulip,grave-w-grave/zulip,cosmicAsymmetry/zulip,rht/zulip,timabbott/zulip,timabbott/zulip,eeshangarg/zulip,showell/zulip,isht3/zulip,arpith/zulip,alliejones/zulip,umkay/zulip,calvinleenyc/zulip,jphilipsen05/zulip,samatdav/zulip,rishig/zulip,cosmicAsymmetry/zulip,synicalsyntax/zulip,rishig/zulip,jackrzhang/zulip,andersk/zulip,peguin40/zulip,susansls/zulip,KingxBanana/zulip,Juanvulcano/zulip,aakash-cr7/zulip,rishig/zulip,amyliu345/zulip,sonali0901/zulip,dawran6/zulip,Diptanshu8/zulip,amyliu345/zulip,christi3k/zulip,krtkmj/zulip,PhilSk/zulip,amyliu345/zulip,KingxBanana/zulip,andersk/zulip,souravbadami/zulip,punchagan/zulip,KingxBanana/zulip,grave-w-grave/zulip,umkay/zulip,paxapy/zulip,jainayush975/zulip,timabbott/zulip,sonali0901/zulip,jrowan/zulip,alliejones/zulip,samatdav/zulip,kou/zulip,karamcnair/zulip,Diptanshu8/zulip,niftynei/zulip,shubhamdhama/zulip,jphilipsen05/zulip,reyha/zulip,Jianchun1/zulip,brainwane/zulip,eeshangarg/zulip,alliejones/zulip,Juanvulcano/zulip,kou/zulip,peiwei/zulip,paxapy/zulip,jainayush975/zulip,vabs22/zulip,synicalsyntax/zulip,samatdav/zulip,zacps/zulip,vabs22/zulip,paxapy/zulip,showell/zulip,Diptanshu8/zulip,verma-varsha/zulip,umkay/zulip,souravbadami/zulip,mohsenSy/zulip,timabbott/zulip,joyhchen/zulip,ryanbackman/zulip,verma-varsha/zulip,jphilipsen05/zulip,JPJPJPOPOP/zulip,j831/zulip,mohsenSy/zulip,TigorC/zulip,sonali0901/zulip,isht3/zulip,sup95/zulip,showell/zulip,niftynei/zulip,hackerkid/zulip,mahim97/zulip,vaidap/zulip,karamcnair/zulip,brockwhittaker/zulip,mahim97/zulip,christi3k/zulip,Diptanshu8/zulip,vaidap/zulip,jrowan/zulip,amanharitsh123/zulip,brockwhittaker/zulip,eeshangarg/zulip,isht3/zulip,dattatreya303/zulip,Galexrt/zulip,susansls/zulip,timabbott/zulip,dhcrzf/zulip,AZtheAsian/zulip,shubhamdhama/zulip,rht/zulip,ryanbackman/zulip,kou/zulip,umkay/zulip,souravbadami/zulip,jainayush975/zulip,dawran6/zulip,rht/zulip,synicalsyntax/zulip,timabbott/zulip,krtkmj/zulip,PhilSk/zulip,vikas-parashar/zulip,Frouk/zulip,andersk/zulip,j831/zulip,karamcnair/zulip,jainayush975/zulip,rishig/zulip,Vallher/zulip,cosmicAsymmetry/zulip,calvinleenyc/zulip,ahmadassaf/zulip,reyha/zulip,j831/zulip,synicalsyntax/zulip,zulip/zulip,JPJPJPOPOP/zulip,jrowan/zulip,sharmaeklavya2/zulip,ryanbackman/zulip,grave-w-grave/zulip,rht/zulip,hackerkid/zulip,amyliu345/zulip,karamcnair/zulip,arpith/zulip,TigorC/zulip,reyha/zulip,Diptanshu8/zulip,blaze225/zulip,vabs22/zulip,samatdav/zulip,Jianchun1/zulip,andersk/zulip,AZtheAsian/zulip,jainayush975/zulip,paxapy/zulip,dawran6/zulip,JPJPJPOPOP/zulip,aakash-cr7/zulip,JPJPJPOPOP/zulip,joyhchen/zulip,zulip/zulip,karamcnair/zulip,zulip/zulip,zacps/zulip,jackrzhang/zulip,tommyip/zulip,brainwane/zulip,hackerkid/zulip,ryansnowboarder/zulip,tommyip/zulip,karamcnair/zulip,dattatreya303/zulip,susansls/zulip,aakash-cr7/zulip,PhilSk/zulip,Jianchun1/zulip,Galexrt/zulip,punchagan/zulip,grave-w-grave/zulip,niftynei/zulip,dattatreya303/zulip,sharmaeklavya2/zulip,rht/zulip,Diptanshu8/zulip,sonali0901/zulip,krtkmj/zulip,Galexrt/zulip