In the buildbot CI step, reset built web files before trying to publish

[srawlins]

These files can have minor differences in them (depending on the version of Dart?), so that it is difficult for them to always be identical to the ones checked in, during CI. Then there is a new warning that pub publish --dry-run produces, if the current git checkout is not clean: dart-lang/pub#4373. We need to reset these files to avoid that warning.

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[srawlins]

CC @dart-lang/analyzer-team