title: Iterating a loop using await or yield causes error
body: <p>I come from the land of <code>Twisted</code>/<a href="https://github.com/twisted/klein" rel="nofollow"><code>Klein</code></a>. I come in peace and to ask for <code>Tornado</code> help.  I'm investigating Tornado and how its take on async differs from Twisted.  Twisted has something similar to <code>gen.coroutine</code> which is <a href="http://twistedmatrix.com/documents/current/core/howto/defer-intro.html#inline-callbacks-using-yield" rel="nofollow"><code>defer.inlineCallbacks</code></a> and I'm able to write async code like this:</p>
  
  <p><em>kleinsample.py</em></p>
  
  <pre><code>@app.route('/endpoint/&lt;int:n&gt;')
  @defer.inlineCallbacks
  def myRoute(request, n):
      jsonlist = []
      for i in range(n):
          yield jsonlist.append({'id': i})
      return json.dumps(jsonlist)
  </code></pre>
  
  <p>curl cmd:</p>
  
  <pre><code>curl localhost:9000/json/2000
  </code></pre>
  
  <p>This endpoint will create a JSON string with <code>n</code> number of elements.  <code>n</code> can be small or very big.  I'm able to break it up in Twisted such that the event loop won't block using <code>yield</code>.  Now here's how I tried to convert this into Tornado:</p>
  
  <p><em>tornadosample.py</em></p>
  
  <pre><code>async def get(self, n):
      jsonlist = []
      for i in range(n):
          await gen.Task(jsonlist.append, {'id': i})    # exception here
      self.write(json.dumps(jsonlist))
  </code></pre>
  
  <p>The traceback:</p>
  
  <pre><code> TypeError: append() takes no keyword arguments
  </code></pre>
  
  <p>I'm confused about what I'm supposed to do to properly iterate each element in the loop so that the event loop doesn't get blocked.  Does anyone know the "Tornado" way of doing this?</p>