Reverts flutter/flutter#144329
Initiated by: goderbauer
Reason for reverting: broke postsubmit doc generation.
Original PR Author: goderbauer

Reviewed By: {devoncarew, HansMuller, gspencergoog}

This change reverts the following previous change:
Original Description:
Dartpad doesn't have a "master" channel anymore, it got renamed to "main". Sadly, specifying "master" is now falling back to "stable" which breaks some of our examples in the docs that require a more current Flutter version, e.g. https://main-api.flutter.dev/flutter/material/TextButton-class.html