message: Make DatasetV2 an abc again.
diff: @@ -54,6 +54,7 @@ from tensorflow.python.util.tf_export import tf_export
  @tf_export("data.Dataset", v1=[])
  +@six.add_metaclass(abc.ABCMeta)
  class DatasetV2(object):
  """Represents a potentially large set of elements.
  