dart_highlight_rules : add `assert` to `keywordControl`

"assert" is a reserved word and a statement in the Dart language (as
opposed to a built-in function call) as described in the spec:

https://www.dartlang.org/docs/spec/latest/dart-language-specification.html#h.cb5i0axenow6
update `tokens_dart.json` regarding `assert` statement
Merge pull request #1918 from rmsmith/master

dart_highlight_rules : add `assert` to `keywordControl`