ESP8266: WebREPL truncates pasted data

I'm running 1.8.4 and I notice that data pasted into the WebREPL is truncated.

Start up the WebREPL and trying pasting the following

"*******************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************"

The result is that only around 256 characters of what is pasted actually shows up. This is most troublesome when trying to use paste mode (CTRL-E) to bring in some existing code.

This might be related to the dupterm ringbuffer getting filled: https://github.com/micropython/micropython/blob/master/esp8266/esp_mphal.c#L39

Start with esp8266/esp_mphal.c at the dupterm ringbuffer reference and reproduce the WebREPL paste-mode case using the long input shown in the issue. Trace where pasted data is buffered and verify that input longer than about 256 characters is received without truncation.