Simplify checkMetaVars $watch statement.

Per the [angular documentation](https://code.angularjs.org/1.2.26/docs/api/ng/type/$rootScope.Scope#$digest) the "magic" hack you are using to detect each digest cycles is unnecessary. Simply register a watch function with no watch expression.
From the documentation:
> If you want to be notified whenever `$digest()` is called, you can register a `watchExpression` function with `$watch()` with no listener.