[babylon] Add support for @@iterator

[bcherny]

Choose one: feature request

Input Code

/* @flow */

declare class A {
  @@iterator(): Iterator<File>;
}

Babel/Babylon Configuration (.babelrc, package.json, cli command)

import { parse } from 'babylon'
import { readFileSync } from 'fs'

let code = readFileSync('file.flow.js', 'utf-8')
let ast = parse(code, { plugins: ['flow'], sourceType: 'module' }) // SyntaxError: Unexpected token

Expected Behavior

@@iterator should be parsed into an Identifier.

Current Behavior

Babylon throws a SyntaxError.

Context

Attempting to use Babylon to parse flow/dom.js (error is thrown on line 55).

Your Environment

software	version(s)
Babel	Master (68476b6)
Babylon	Master (68476b6)
node	8.1.3
npm	5.3.0
Operating System	OSX

[babel-bot]

Hey @bcherny! We really appreciate you taking the time to report an issue. The collaborators
on this project attempt to help as many people as possible, but we're a limited number of volunteers,
so it's possible this won't be addressed swiftly.

If you need any help, or just have general Babel or JavaScript questions, we have a vibrant Slack
community that typically always has someone willing to help. You can sign-up here
for an invite.

[rajasekarm]

This should be a small change. I'll give a PR today.