@Override
    public IncrementalExtensionType incrementalType(ProcessingEnvironment processingEnvironment) {
      return IncrementalExtensionType.ISOLATING;
    }
Tests of Gradle incrementality in the presence of extensions. See <a
href="https://docs.gradle.org/5.0/userguide/java_plugin.html#sec:incremental_annotation_processing">
incremental annotation processing</a> in the Gradle user guide.
value/src/test/java/com/google/auto/value/processor/IncrementalExtensionTest.java