Declare a global which may or may not be present

I'm trying to do something like `declare var foo? : string` such that `foo` is either a string, or doesn't exist at all as a global variable.

I can do `declare var foo : string | void` but that only gets me half way there; `console.log(foo)` will still typecheck. What I want is force the code to have to do a `typeof` check before using the variable, to make sure it exists.

Is there any way to declare a global that might not exist, in flow?