private void warning(String msg, Element element, AnnotationMirror annotation) {
    processingEnv.getMessager().printMessage(Kind.WARNING, msg, element, annotation);
  }
Returns the contents of a {@code Class[]}-typed "value" field in a given {@code
annotationMirror}.
service/processor/src/main/java/com/google/auto/service/processor/AutoServiceProcessor.java