Revert "Balance tests equally across shards."

This reverts commit f827eb3a78e3b3ff8352ece9f35ed94b52e1d917.

Reason for revert: The tests being run, and the results being compared to, are changing dramatically, causing all sorts of test results going from [status] -> Skipped and from New Test -> [status].

Something is really going wrong.

Original change's description:
> Balance tests equally across shards.
>
> The sharded test runner invocations are now passed the previous results
> which contains the test timing, which are used to simulate how long each
> shard would take to run. The shards are now balanced as evenly as
> possible on a test level, taking multiple cores into account.
>
> Sharded tests are now run starting with the slowest test first, such
> that extremely long running tests finish as early as possible. This
> behavior ensures the cores are saturated and can be padded with fast
> tests near the end, rather than waiting for a few slow tests to complete
> while the rest of the system is idle.
>
> The algorithm works very well whenever it's able to accurately predict
> the time to run shards. In a number of cases, the model doesn't quite
> reflect reality and the data, which makes it fairly imperfect but still
> reasonably good. I think a second order feedback loop might kick in once
> it reorders the tests across shards and the test timing data reflects
> the new test timings.
>
> Multitests are no longer always sent to the same shard, since the data
> isn't available at the moment, and the change as-is speeds up the test
> running considerably.
>
> The front end unit test suites currently ignore the feature as there are
> no benefits yet to improving those quick shards.
>
> Upgrade the language version to 3.0.0 so patterns can be used and fix
> a mixin not being a mixin.
>
> Fixes: b/291585137
> Change-Id: I3cc1b1d96038d5b46e836b091e299097717c226c
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/314081
> Reviewed-by: William Hesse <whesse@google.com>
> Commit-Queue: Jonas Termansen <sortie@google.com>

Change-Id: I233e4bfa6d6ecf0cea4f97c1e47f1635f7b9040c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/315060
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: William Hesse <whesse@google.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>