"internal server error" and "error: 500" are EXTREMELY strong clues that the problem is server-side.
there aren't a lot of HTTP error codes, and it's worth remembering the most common ones, like the 200, 300, 400 series and 500 itself. will help you troubleshooting later.
flutter channel master
And it kept failing with:
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git: remote: Internal Server Error.
git: remote:
git: fatal: unable to access 'https://github.com/flutter/flutter.git/': The requested URL returned error: 500
Switching channels failed with error code 128.
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thought I was doing something wrong and spent some time troubleshooting.