[gardening] Add optional typename to fix flutter build.

Follow-up to https://dart.googlesource.com/sdk/+/74846b96c394e78c37060d8be7c350fa8c1b6a6d

TEST=flutter ci

Change-Id: I31cdfe067d65f7cc39912c58ab62e0f9a8157e32
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/436640
Reviewed-by: Derek Xu <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam

runtime/vm/lockers.cc

@@ -72,7 +72,7 @@ void SafepointLocker<M>::AcquireLock() {
 }
 
 template <typename M>
-M::WaitResult SafepointLocker<M>::Wait(int64_t millis) {
+typename M::WaitResult SafepointLocker<M>::Wait(int64_t millis) {
   Thread* thread = Thread::Current();
   if (thread != nullptr) {
     typename M::WaitResult result;

runtime/vm/lockers.h

@@ -279,7 +279,7 @@ class SafepointLocker : public ValueObject {
   explicit SafepointLocker(M* monitor) : monitor_(monitor) { AcquireLock(); }
   virtual ~SafepointLocker() { ReleaseLock(); }
 
-  M::WaitResult Wait(int64_t millis = Monitor::kNoTimeout);
+  typename M::WaitResult Wait(int64_t millis = Monitor::kNoTimeout);
 
   void NotifyAll() { monitor_->NotifyAll(); }