Allow for recursive invocation of SafepointOperationScopes.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index eee6d049a5dfdf039ba1fcd1549e764968d2b206..d43f494ad172c2c8e08057f2c6531c572a31cffc 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -2527,7 +2527,7 @@ Thread* Isolate::ScheduleThread(bool is_mutator, bool bypass_safepoint) {
 
     // If a safepoint operation is in progress wait for it
     // to finish before scheduling this thread in.
-    while (!bypass_safepoint && safepoint_handler()->safepoint_in_progress()) {
+    while (!bypass_safepoint && safepoint_handler()->SafepointInProgress()) {
       ml.Wait();
     }