builder.add(invokable);
      }
    }
    return new FactoryMethodReturnValueTester(cls, builder.build(), "public static methods");
  }

  /** Runs sanity tests against return values of static factory methods declared by a class. */
  public final class FactoryMethodReturnValueTester {
    private final Set<String> packagesToTest = new HashSet<>();
    private final Class<?> declaringClass;