title: Issue building Gradle project: error message "functional test classes of type org.gradle.api.internal.tasks.DefaultSourceSetOutput"
body: <p>I'm trying to build a Gradle project but when I build the project with <code>gradle clean build</code> I get the following error message:</p>
  
  <blockquote>
    <p>Could not get unknown property 'classesDirs' for functional test classes of type org.gradle.api.internal.tasks.DefaultSourceSetOutput</p>
  </blockquote>
  
  <p>How can I fix my build?</p>