title: How can I filter a map / access the entries
body: <p>The Map interface doesn't seem to provide access to the entries as an iterable, nor does it expose a where method to filter entries. Am I missing something? Is there a simple workaround?</p>
  
  <p>e.g.</p>
  
  <pre class="lang-dart prettyprint-override"><code>Map map;
  final filteredMap = map.where((k, v) =&gt; k.startsWith("foo"));
  </code></pre>