title: Filter and get a new Map from ' Map<String,ProductItem>myMap' in dart
body: <p>I have a Map in dart as </p>
  
  <pre><code>Map&lt;String, List&lt;ProductItem&gt;&gt; _productItemsMap;
  </code></pre>
  
  <p>I want to filter and get a new <code>Map&lt;String, List&lt;ProductItem&gt;&gt; _newMap</code>
  which has the Product Item's id the same as my condition. 
  How can I do that in dart?
  Thanks for you help.</p>