[benchmarks] Use gen_kernel instead of dart2js for isolate group benchmarks.

 - dart2js does not run in NNBD but gen_kernel does.
 - Changes to dart2js to not trigger VM benchmarks to run on Golem, allowing performance differences to accumulate until the next unrelated VM change. Changes to the front end or package:vm cause a binary diff in the platform file or kernel-service snapshot and so do trigger VM benchmarks to run on Golem.
 - These benchmarks don't care about running dart2js in particular, just some large program.

Change-Id: I85e339c32edbf9d0261081623378475fa0b9507a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/270022
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Jonas Termansen <sortie@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>