code: public static <T> Set<Annotation> getQualifiers(Key<T> key) {
      Annotation annotation = key.getAnnotation();
      if (annotation != null) {
        
        // Replace 'com.google.inject.name.Named' with 'javax.inject.Named' 
        if (annotation instanceof com.google.inject.name.Named) {
          annotation = toThreeThirtyNamed((com.google.inject.name.Named)annotation);
        }
        
        Class<? extends Annotation> type = annotation.annotationType();
        if (type.isAnnotationPresent(Qualifier.class)) {
          return Collections.singleton(annotation);
        }
        
        return Collections.<Annotation>singleton(new GuiceQualifier<>(key));
      }
      
      Class<? extends Annotation> annotationType = key.getAnnotationType();
      if (annotationType != null) {
        return Collections.<Annotation>singleton(new GuiceQualifier<>(key));
      }
      
      return Collections.emptySet();
    }
docs: Returns a {@link Set} of qualifier {@link Annotation}s for
  the given {@link Key}.
  
  NOTE: The {@link Set} will be either empty or will have
  at most one {@link Annotation} in it.