Support for optional chaining

[pronebird]

Flow does not understand optional chaining from babel 7 alpha, but it should.

Reference: babel/babel#5813

While this is stage-1 still and experimental a failure to support it will definitely make it difficult to adopt the new proposal and continue using flow at the same time.

I hope it's a matter of upgrading babel and tagging @next release.

Example:

class AccountInput extends Component {
  _ref: ?HTMLInputElement;
  focus() {
    this._ref?.focus();
//            ^ Unexpected token .
  }

}

[pronebird]

which is #4303