This reverts commit 5585ed99039efb3705025e1c58170cdb86af111b.
Additionally, the following _flutter.runInView deadlock is fixed.
Previously, a deadlock would occur when service protocol
_flutter.runInView is used to restart the engine wihtout tearing down
the shell: the shared mutex of the service protocol will be locked
during the restart as it's in the middle of handling a service protocol
message; if ServiceProtocol::AddHandler is also called during the
restart, the deadlock happens as AddHandler also requires such lock.
test/integration.shard/background_isolate_test.dart would fail
without this fix.