Use require("recast").types.builtInTypes for stricter assertions

In particular, the `isObject.{check,assert}` functions require that the argument is non-`null`, by using `Object.prototype.toString` to check the internal class of the value.

Where @spicyj previously added null checks in `findChildReprints`, I have gone a step further and asserted the same thing, since I believe that situation should never arise after the other changes in this commit.