title: WeakMap polyfill throwing error when trying to define property on inextensible object
body: <p>The <a href="https://github.com/webcomponents/webcomponentsjs/blob/master/src/WeakMap/WeakMap.js" rel="nofollow">WeakMap polyfill</a> is throwing an error when trying to define property on inextensible object.
  Those are in the middle of a bunch of node, javascript code and libraries so I couldn't actually point out where causes the issue. There are many other libraries which also have their own polyfill. It's difficult to debug in which library causes the error. And, the errors are ONLY on IE10.<br>
  To get rid of it, I added a checker before defining a property like (line 26 in the above file):</p>
  
  <pre><code>if (entry &amp;&amp; entry[0] === key) {
      entry[1] = value;
  }
  else if (Object.isExtensible(key)) {
      defineProperty(key, this.name, {
          value: [ key, value ],
          writable: true
      }); 
  }
  </code></pre>
  
  <p>My question is, Is it safe/right to fix it as in my code above? If not, how should I fix my problem? </p>