Tweaks to dart_test. A failing test I'm struggling with...

The first commit just pulls the creation of the grammar into a top-level field. Instantiated when first used. A good best-practice.

The 2nd commit adds test for NUMBER (which pass) and singleLineString (which fail).

I'd love to use this grammar, but the failure of SLS is driving me nuts. Tried to make progress.

Curious if you have a sec to debug and see anything obvious.

Thanks!