Avoid using += with let variables to avoid let compound assigment deopt
Avoid using += with let variables to avoid let compound assigment deopt

See https://jsperf.com/let-compound-assignment

CraZySacX/atom,stinsonga/atom,FIT-CSE2410-A-Bombs/atom,FIT-CSE2410-A-Bombs/atom,atom/atom,Mokolea/atom,liuderchi/atom,decaffeinate-examples/atom,PKRoma/atom,stinsonga/atom,CraZySacX/atom,kevinrenaers/atom,kevinrenaers/atom,atom/atom,stinsonga/atom,brettle/atom,Arcanemagus/atom,AlexxNica/atom,sotayamashita/atom,AlexxNica/atom,t9md/atom,sotayamashita/atom,andrewleverette/atom,liuderchi/atom,decaffeinate-examples/atom,atom/atom,PKRoma/atom,liuderchi/atom,brettle/atom,Mokolea/atom,decaffeinate-examples/atom,brettle/atom,decaffeinate-examples/atom,kevinrenaers/atom,CraZySacX/atom,Arcanemagus/atom,andrewleverette/atom,liuderchi/atom,Arcanemagus/atom,sotayamashita/atom,t9md/atom,Mokolea/atom,stinsonga/atom,AlexxNica/atom,t9md/atom,ardeshirj/atom,PKRoma/atom,andrewleverette/atom,FIT-CSE2410-A-Bombs/atom,ardeshirj/atom,ardeshirj/atom