[analytics] migrate to core Dart Plugin reporting

[pq]

Dart 502 introduces the core analytics implementation and this migrates our stub implementations to use it. We remove the now defunct UnifiedAnalytics implementation to keep things tidy.

Note that this does introduce more version coupling. Once published we'll have more places that will break in the presence of a pre-502 Dart plugin. (Since this was already happening with the PubRoot cleanup, e.g., #8725, I didn't consider doing version testing and any defensive programming; if folks feel strongly we can revisit though!)

Follow-up from #8598

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[helin24]

Great! I agree we probably don't need to worry about the old Dart version issue.

[pq]

Cool, thanks! I'm a little surprised that IDEA doesn't support explicit version dependencies but since they don't I think occasional issues with version skew are probably tolerable (if not expected). Worth keeping an eye out though!