Track type of this in callbacks #968

There's an error (marked with <---) in this snippet which Flow could potentially catch:

class RemoteFile {
  url: string;
...
  promiseXHR(xhr: XMLHttpRequest): Q.Promise<[any, Event]> {
    var url = this.url;
    var deferred = Q.defer();
    xhr.addEventListener('load', function(e) {
      if (this.status >= 400) {
        deferred.reject(this.status + ' ' + this.statusText);
      } else {
        deferred.resolve([this.response, e]);
      }
    });
    xhr.addEventListener('error', function(e) {
      deferred.reject(`Request for ${this.url} failed: ${this.status}`);  // <---
    });
    this.numNetworkRequests++;
    xhr.send();
    return deferred.promise;
  }
}

The problem is that this inside the addEventListener callback refers to the XMLHttpRequest object (I believe) and not the RemoteFile class. The latter has a url property but not a status property, whereas the former has status but not url. Clearly this code isn't right!

Flow doesn't complain, though. It thinks this has type any in the event listener.

Where possible, the DOM declarations should indicate the type of this in callbacks to prevent this sort of mistake.

For example, Google's Closure Compiler provides an @this annotation. You can see an example of this in action in the declaration for Array.prototype.find.