d2bc055651
This brings Mutex and ConditionVariable classes to guard access to shared variables. TEST=shared_test, shared_primitives_test BUG=https://github.com/dart-lang/sdk/issues/55991 CoreLibraryReviewExempt: dart:concurrent library is experimental, restricted to main/dev channels, vm-only at this point. Change-Id: I64ca7a0434cbe0079976133ccbf9de2dd4c0d7b1 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/372181 Commit-Queue: Alexander Aprelev <aam@google.com> Reviewed-by: Slava Egorov <vegorov@google.com>