title: dart: unnecessary_null_comparision
body: <p>For the following code:</p>
  <pre><code>bool assertTest(int? n1, int? n2) {
    return (n1 == null) || (n1 != null &amp;&amp; n2 != null);
  }
  </code></pre>
  <p>there is a warning at <code>n1 != null</code>  saying <code>The operand can't be null, so the condition is always true.</code>
  Why does this warning show up? n1 is obviously nullable.</p>