Construct a class object for a protobuf described by descriptor.

  Composite descriptors are handled by defining the new class as a member of the
  parent class, recursing as deep as necessary.
  This is the dynamic equivalent to:

  class Parent(message.Message):
    __metaclass__ = GeneratedProtocolMessageType
    DESCRIPTOR = descriptor
    class Child(message.Message):
      __metaclass__ = GeneratedProtocolMessageType
      DESCRIPTOR = descriptor.nested_types[0]

  Sample usage:
    file_descriptor = descriptor_pb2.FileDescriptorProto()
    file_descriptor.ParseFromString(proto2_string)
    msg_descriptor = descriptor.MakeDescriptor(file_descriptor.message_type[0])
    msg_class = reflection.MakeClass(msg_descriptor)
    msg = msg_class()

  Args:
    descriptor: A descriptor.Descriptor object describing the protobuf.
  Returns:
    The Message class object described by the descriptor.

def MakeClass(descriptor):
  """Construct a class object for a protobuf described by descriptor.

  Composite descriptors are handled by defining the new class as a member of the
  parent class, recursing as deep as necessary.
  This is the dynamic equivalent to:

  class Parent(message.Message):
    __metaclass__ = GeneratedProtocolMessageType
    DESCRIPTOR = descriptor
    class Child(message.Message):
      __metaclass__ = GeneratedProtocolMessageType
      DESCRIPTOR = descriptor.nested_types[0]

  Sample usage:
    file_descriptor = descriptor_pb2.FileDescriptorProto()
    file_descriptor.ParseFromString(proto2_string)
    msg_descriptor = descriptor.MakeDescriptor(file_descriptor.message_type[0])
    msg_class = reflection.MakeClass(msg_descriptor)
    msg = msg_class()

  Args:
    descriptor: A descriptor.Descriptor object describing the protobuf.
  Returns:
    The Message class object described by the descriptor.
  """
  if descriptor in MESSAGE_CLASS_CACHE:
    return MESSAGE_CLASS_CACHE[descriptor]

  attributes = {}
  for name, nested_type in descriptor.nested_types_by_name.items():
    attributes[name] = MakeClass(nested_type)

  attributes[GeneratedProtocolMessageType._DESCRIPTOR_KEY] = descriptor

  result = GeneratedProtocolMessageType(
      str(descriptor.name), (message.Message,), attributes)
  MESSAGE_CLASS_CACHE[descriptor] = result
  return result