Amend type annotation fields to core node types in fb-harmony.js

This is likely to change a bit in the future (the flow parser doesn't use the exact same AST structure as what esprima-fb has right at the moment), but for now we need to understand these fields so we can build out a Recast transform that strips Flow type annotations
I'm a noob to adding ast-types, so not sure what you mean by "default function"?

The reason `typeAnnotation` is `obj|undefined` is (as you pointed out previously) to enable us to fork the harmony branch without having to update upstream tests every time we sync them down to include the nulled out new leaf nodes