title: The SourceSet 'commonTest' is not recognized by the Android Gradle Plugin. Perhaps you misspelled something?
body: <p>Seemingly the same root cause as <a href="https://stackoverflow.com/questions/49511200/the-sourceset-instrumenttest-is-not-recognized-by-the-android-gradle-plugin">this question</a>, but the answer there will not work for me. I have a custom source set called <code>commonTest</code> that I use for sharing certain test utility code across the <code>test</code> and <code>androidTest</code> source sets. Here is my relevant project config:</p>
  
  <pre><code>sourceSets {
      // This lets us write test utility code that can be used by both unit tests and android tests
      commonTest {
          java
      }
      test {
          java.srcDirs += commonTest.java.srcDirs
      }
      androidTest {
          java.srcDirs += commonTest.java.srcDirs
      }
  }
  </code></pre>
  
  <p>This worked fine with AGP 3.0.1, but breaks on AGP 3.1.0. <strong>Are custom source sets no longer supported?</strong></p>