[vm, gc] Avoid growth policy data race.

TEST=tsan
Bug: https://github.com/dart-lang/sdk/issues/61862
Change-Id: Icd16140999282b33fd9fbcceda901b82e4267c7a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/458421
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/vm/dart_api_impl.cc

@@ -6008,6 +6008,9 @@ DART_EXPORT Dart_Handle Dart_FinalizeLoading(bool complete_futures) {
   // If this is an auxiliary isolate inside a larger isolate group, we will not
   // re-initialize the growth policy.
   if (I->group()->ContainsOnlyOneIsolate()) {
+    // Helper tasks like the background compile might be running. Avoid data
+    // races on the growth policy.
+    GcSafepointOperationScope safepoint(T);
     I->group()->heap()->old_space()->EvaluateAfterLoading();
   }