How do I strip my code of type annotations from Flow?

Flow is Facebook's type checking system for JavaScript.

Here's an example of flow syntax you're supposed to put directly in your JS files:

type Message = {
  id: string;
  threadID: string;
  authorName: string;
  date: Date;
  text: string;
  isRead: boolean;
};
// ...
get: function(id: string): ?Message {
  return _messages[id];
},

This seems like the sort of thing that should be obvious in the docs, but I don't see anything about it: where is the step when my JS files go back to being pure JS files???