:art: Use ES6 Set instead of Array for LinterViews::_messages
:art: Use ES6 Set instead of Array for LinterViews::_messages

AtomLinter/Linter,iam4x/linter,e-jigsaw/Linter,elkeis/linter,AsaAyers/linter,JohnMurga/linter,blakeembrey/linter,josa42/Linter,mdgriffith/linter,kaeluka/linter,levity/linter,UltCombo/linter,DanPurdy/linter,Arcanemagus/linter,atom-community/linter,steelbrain/linter,shawninder/linter