Clone latency.dart into the benchmark from EventLoopLatencyJson
Change-Id: I92090b4decf49e67b937d8095bb7c36adf79c509
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/210430
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>