[Flow] Arrow function type parameter declarations

[gabelevi]

In Flow, you can declare type parameters for arrow functions. The syntax looks like this:

  const arrowFunctionWithoutTypeParams = (x) => x;
  const arrowFunctionWithTypeParams = <T>(x: T): T => x;

This is a little tricky to parse, especially with JSX. This PR attempts to solve the problem as follows

1. Try to parse the expression as if it were JSX. JSX is probably more common and this will lead to less backtracking. If this fails, save the error. If the JSX plugin isn't in use, then skip this step.
2. If the next token is <, then parse a type parameter declaration. Then parse an assignment expression. If that expression turns out to an ArrowFunctionExpression, then attach those type params to that expression. Otherwise, fail. On failure throw the JSX error, if it exists, or a new error.
3. If this is not JSX And doesn't start with < then just parse an assignment expression as usual.

This was further complicated by the need to reinterpret tokens and mess with context. But at least it seems to work :)

[gabelevi]

This fixes https://phabricator.babeljs.io/T7330

[sebmck]

This isn't super obvious but cloning state is actually super expensive, for example there's a tokens array on there that gets cloned and this can sometimes number in the tens of thousands for really large files.

[sebmck]

Oh great, just noticed that state is only used within the JSX if, you can just move it into there. It should resolve the more pathological cases but the perf issue will still be there. We should really use babylon to use immutable data structures so this state cloning is really cheap.

[gabelevi]

Sure, I'll move it in there. It will still be hit for every JSX expression, I'm afraid, but that's better than every expression :)

[sebmck]

Thanks @gabelevi! I'll publish a new release now.

[hzoo]

We aren't running the changes on babel in travis yet right?

[sebmck]

@hzoo Nope.

[sebmck]

I pushed a new release just now, I'll run make test-babel to ensure that I haven't broken anything...

[hzoo]

Ok cool - it does suck that it would make the tests run a lot longer but worth doing. (installing it now locally too, seeing no issues)