title: No unused expressions in mocha chai unit test using standardJS
body: <p>I am using standardJS for testing my code. Also I am using mocha chai. With that I'm getting the error <code>expect an assignment or function call and instead saw an expression.</code> for this line:</p>
  
  <pre><code>expect(err).to.be.null
  </code></pre>
  
  <p>But this line is correct, so what am I doing wrong?</p>