Scope constructor `instanceof` check

When writing Jest unit tests for a jscodeshift codemod (https://github.com/cpojer/js-codemod/pull/2) the [Scope constructor `instanceof` check](https://github.com/benjamn/ast-types/blob/master/lib/scope.js#L14) fails, I haven't fully tracked down whats causing it (maybe related to running in a child process) but i can see the transform is working fine locally via jscodeshift or via http://astexplorer.net/#/LDGg3pg5ne

What is the purpose of this check? Im wondering if it can be achieved another way? i'm happy to do a PR if you're cool with changing it.

cc @cpojer @benjamn

Start with the Scope constructor `instanceof` check in lib/scope.js at line 14 and reproduce the failure in the Jest unit tests for the jscodeshift codemod. Compare that behavior with running the transform through jscodeshift and astexplorer.net. Done means the purpose and cause of the failing check are established and the test behavior is resolved without breaking the transform.

Written by the indexing model from the issue text.