[javascript mode] Improve handling of async in object literal
[javascript mode] Improve handling of async in object literal

Closes #4152

codio/CodeMirror,grzegorzmazur/CodeMirror,rrandom/CodeMirror,kerabromsmu/CodeMirror,thomasjm/CodeMirror,wingify/CodeMirror,pabloferz/CodeMirror,crazy4chrissi/CodeMirror,SidPresse/CodeMirror,MarcelGerber/CodeMirror,jkaplon/CodeMirror,TDaglis/CodeMirror,SidPresse/CodeMirror,sourcelair/CodeMirror,sourcelair/CodeMirror,mtaran-google/CodeMirror,hackmdio/CodeMirror,TDaglis/CodeMirror,namin/CodeMirror,jkaplon/CodeMirror,sourcelair/CodeMirror,hackmdio/CodeMirror,ficristo/CodeMirror,kerabromsmu/CodeMirror,xeronith/CodeMirror,codio/CodeMirror,xeronith/CodeMirror,schanzer/CodeMirror,jayaprabhakar/CodeMirror,notepadqq/CodeMirror,INTELOGIE/CodeMirror,kerabromsmu/CodeMirror,mtaran-google/CodeMirror,wingify/CodeMirror,schanzer/CodeMirror,adobe/CodeMirror2,nightwing/CodeMirror,wingify/CodeMirror,adobe/CodeMirror2,thomasjm/CodeMirror,ficristo/CodeMirror,adobe/CodeMirror2,MarcelGerber/CodeMirror,ejgallego/CodeMirror,ejgallego/CodeMirror,hackmdio/CodeMirror,rrandom/CodeMirror,notepadqq/CodeMirror,dperetti/CodeMirror,rrandom/CodeMirror,SidPresse/CodeMirror,INTELOGIE/CodeMirror,INTELOGIE/CodeMirror,alur/CodeMirror,MarcelGerber/CodeMirror,pabloferz/CodeMirror,ficristo/CodeMirror,grzegorzmazur/CodeMirror,thomasjm/CodeMirror,schanzer/CodeMirror,xeronith/CodeMirror,TDaglis/CodeMirror,alur/CodeMirror,alur/CodeMirror,nightwing/CodeMirror,nightwing/CodeMirror,codio/CodeMirror,namin/CodeMirror,dperetti/CodeMirror,dperetti/CodeMirror,jayaprabhakar/CodeMirror,grzegorzmazur/CodeMirror,jkaplon/CodeMirror,crazy4chrissi/CodeMirror,namin/CodeMirror,ejgallego/CodeMirror,jayaprabhakar/CodeMirror,mtaran-google/CodeMirror,notepadqq/CodeMirror,crazy4chrissi/CodeMirror,pabloferz/CodeMirror