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
The convention in the Mozilla/Esprima AST for possibly missing fields is to make them `null`able. This is important here because the default function will be called if a field's value is `undefined`, even if the field is actually present. So the default function would be called unnecessarily for every absent `.typeAnnotation`.