add flow exceptions for polymorphic types - Ref #109

[hzoo]

For

#109 (comment)

export function foo<X>(x: X): X {
  return x;
}

3:20 error "X" is not defined no-undef

Also for mixed by not visiting those 2 cases. Fixed in babel/babel#1669

[hzoo]

And since there are a lot of globals (Array), etc maybe we can import the definitions in https://github.com/facebook/flow/tree/master/lib core.js?

[hzoo]

@sebmck this look good? There probably doesn't need to be a PolymorphicTypeAnnotation right?

[hzoo]

good to merge?