net/http: Flush in TimeoutHandler for Go1.13 is broken and might need a revert [1.13 backport]

[gopherbot]

@bcmills requested issue #34439 to be considered for backport to the next 1.13 minor release.

@gopherbot, please backport to Go 1.13: we should minimize the window during which the erroneous method is present.

[bcmills]

In #34439 (comment), @pam4 notes:

timeoutWriter is unexported, so no compile-time incompatibility, and although the method exists, the interface was never actually implemented. Go 1.13 docs made a false/impossible claim and I think we should just take it back. […] any program using the method is already broken

In #34439 (comment), @bradfitz replies:

I'm fine reverting the Flusher stuff, though. That's easier than making it work properly.

[bcmills]

Based on the above, approving for Go 1.13.2.

[gopherbot]

Change https://golang.org/cl/197543 mentions this issue: [release-branch.go1.13] net/http, doc/go1.13.html: revert TimeoutHandler.Flush

[gopherbot]

Closed by merging de96471 to release-branch.go1.13.