[dart mode] Add "extension" and "on" keywords
[dart mode] Add "extension" and "on" keywords

This is for Dart 2.6 extension method syntax
pabloferz/CodeMirror,nightwing/CodeMirror,wingify/CodeMirror,wingify/CodeMirror,dperetti/CodeMirror,codio/CodeMirror,dperetti/CodeMirror,hackmdio/CodeMirror,codio/CodeMirror,codio/CodeMirror,nightwing/CodeMirror,wingify/CodeMirror,hackmdio/CodeMirror,dperetti/CodeMirror,pabloferz/CodeMirror,pabloferz/CodeMirror,nightwing/CodeMirror,hackmdio/CodeMirror