Perform the actual sending to platform. This is separated from
        `flush()` since it needs to be inside a middleware call.

async def _flush(self, request: 'Request', stacks: List[Stack]):
        """
        Perform the actual sending to platform. This is separated from
        `flush()` since it needs to be inside a middleware call.
        """

        for stack in stacks:
            await self.platform.send(request, stack)