title: TypeError: undefined is not a function when copying Map
body: <p>Why is this code:</p>
  
  <pre><code>let a = {};
  let m = new Map(a);
  </code></pre>
  
  <p>giving me the error:</p>
  
  <pre><code>code.0.0.128.js:33943 Uncaught TypeError: undefined is not a function
      at new Map (native)
      at http://localhost:/code.0.0.128.js:33943:26
      at exports.default (http://localhost:/code.0.0.128.js:34006:7)
      at combination (http://localhost:/code.0.0.128.js:28672:30)
      at i (&lt;anonymous&gt;:1:14471)
      at u (&lt;anonymous&gt;:1:14757)
      at &lt;anonymous&gt;:1:17652
      at Object.dispatch (http://localhost:/code.0.0.128.js:28238:23)
      at dispatch (&lt;anonymous&gt;:1:18094)
      at http://localhost:/code.0.0.128.js:29398:17
  </code></pre>
  
  <p>If I try this I get no error:</p>
  
  <pre><code>let m = new Map();
  </code></pre>