title: What is the equivalent of this python dictionary in Dart?
body: <p>What is the equivalent of this python dictionary in Dart?</p>
  
  <pre><code>edges = {(1, 'a') : 2,
           (2, 'a') : 2,
           (2, '1') : 3,
           (3, '1') : 3}
  </code></pre>