title: What's the benefit of factory constructor in MapEntry class?
body: <p>Original code for <a href="https://api.dart.dev/stable/2.10.4/dart-core/Map-class.html" rel="nofollow noreferrer"><code>MapEntry</code></a></p>
  <pre class="lang-dart prettyprint-override"><code>class MapEntry&lt;K, V&gt; {
    final K key;
    final V value;
  
    const factory MapEntry(K key, V value) = MapEntry&lt;K, V&gt;._;
  
    const MapEntry._(this.key, this.value);
  }
  </code></pre>
  <p>What's the need to create above <code>factory</code> constructor when you can simply have:</p>
  <pre class="lang-dart prettyprint-override"><code>class MapEntry&lt;K, V&gt; {
    final K key;
    final V value;
  
    const MapEntry(this.key, this.value);
  }
  </code></pre>