Simplify Array<T>|IArrayLike<T> to IArrayLike<T> in externs/es6.js
Simplify Array<T>|IArrayLike<T> to IArrayLike<T> in externs/es6.js

After 9d52fed2c7da2a9e3807477c852c60b56edd2497, Array implements
IArrayLike, so the union type can be simplified.

ChadKillingsworth/closure-compiler,mprobst/closure-compiler,selkhateeb/closure-compiler,MatrixFrog/closure-compiler,anomaly/closure-compiler,tdelmas/closure-compiler,google/closure-compiler,redforks/closure-compiler,google/closure-compiler,anomaly/closure-compiler,mprobst/closure-compiler,vobruba-martin/closure-compiler,monetate/closure-compiler,nawawi/closure-compiler,LorenzoDV/closure-compiler,brad4d/closure-compiler,Medium/closure-compiler,shantanusharma/closure-compiler,brad4d/closure-compiler,Dominator008/closure-compiler,tiobe/closure-compiler,tdelmas/closure-compiler,LorenzoDV/closure-compiler,GerHobbelt/closure-compiler,Yannic/closure-compiler,Yannic/closure-compiler,Pimm/closure-compiler,monetate/closure-compiler,nawawi/closure-compiler,redforks/closure-compiler,tiobe/closure-compiler,Pimm/closure-compiler,GerHobbelt/closure-compiler,GerHobbelt/closure-compiler,superkonduktr/closure-compiler,shantanusharma/closure-compiler,mprobst/closure-compiler,google/closure-compiler,tiobe/closure-compiler,shantanusharma/closure-compiler,ralic/closure-compiler,vobruba-martin/closure-compiler,google/closure-compiler,Medium/closure-compiler,superkonduktr/closure-compiler,ChadKillingsworth/closure-compiler,Medium/closure-compiler,ChadKillingsworth/closure-compiler,tdelmas/closure-compiler,brad4d/closure-compiler,Pimm/closure-compiler,MatrixFrog/closure-compiler,superkonduktr/closure-compiler,vobruba-martin/closure-compiler,MatrixFrog/closure-compiler,monetate/closure-compiler,ralic/closure-compiler,Yannic/closure-compiler,superkonduktr/closure-compiler,ralic/closure-compiler,GerHobbelt/closure-compiler,anomaly/closure-compiler,nawawi/closure-compiler,Yannic/closure-compiler,redforks/closure-compiler,selkhateeb/closure-compiler,vobruba-martin/closure-compiler,tdelmas/closure-compiler,tiobe/closure-compiler,Dominator008/closure-compiler,LorenzoDV/closure-compiler,monetate/closure-compiler,mprobst/closure-compiler,shantanusharma/closure-compiler,selkhateeb/closure-compiler,Dominator008/closure-compiler,anomaly/closure-compiler,nawawi/closure-compiler,ChadKillingsworth/closure-compiler,MatrixFrog/closure-compiler