This CL also removes ability to assign names to mutexes which was
added in c25ebfff96 but did not yield
any interesting data.
TEST=ci
CoreLibraryReviewExempt: Changes to dart:concurrent only.
Cq-Include-Trybots: luci.dart.try:vm-fuchsia-release-x64-try,vm-win-debug-x64-try,vm-win-release-x64-try,vm-aot-linux-product-x64-try
Change-Id: Id41e1d29832f6008e02f0a571ee67564e1a84224
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/375300
Reviewed-by: Martin Kustermann <kustermann@google.com>
Vyacheslav Egorov