Don't die when patching against a null node

See facebook/regenerator#103.

Test Plan:
npm test
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.
Merge pull request #1 from benjamn/patch-null-additional-assertions

Use require("recast").types.builtInTypes for stricter assertions
Merge pull request #74 from spicyj/patch-null

Don't die when patching against a null node.