Hate to be that guy but, Flow for JS is something like that. You can optionally enable it and even enable/disable it in certain files, functions, lines, etc.
That is optional typing. The author is talking about gradual or progressive typing.
But he mentions he wishes he could start dynamically and end statically. By enabling Flow at the end, don't you basically achieve that? You can also enable it progressively for files as you move on, and still rapidly prototype without it. In fact, unless I'm mistaken I believe it's "opt-in" by default, ie. Flow won't scan the file unless you include the special comment keyword at the top of the file.
Could you explain the difference? I though TypeScript was a good example of gradual typing.