title: Dart sum list of lists by element
body: <p>I'm trying to add all elements in a list of lists by index and average them:</p>
  <pre><code>List&lt;List&lt;double&gt;&gt; data = [[1.0, 2.0, 3.0], [2.0, 3.0, 5.0], [8.0, 7.0, 2.0]]
  </code></pre>
  <p>The result should be a list of doubles each summed up and then divided by the total lengths of above data:</p>
  <pre><code>[11.0, 12.0, 10.0] // divide these by 3
  [3.67, 4.0, 3.33] // should be the result
  </code></pre>
  <p>What is the best way to do this? This is taking too long in my flutter app (actual data list contains 60 lists of 2000 doubles each) and yes, I am also asserting the length of each list is equal. and is not empty or having unexpected data types.</p>