[vm, gc] Require a safepoint for heap iteration.

runtime/vm/compiler_stats.cc

Index: runtime/vm/compiler_stats.cc
diff --git a/runtime/vm/compiler_stats.cc b/runtime/vm/compiler_stats.cc
index 647f70bf3cd4b6f8a303314e52e0c0169ade106d..3493ca5ecdd5a9a8fbeadb0097ad1ae2a06f29c7 100644
--- a/runtime/vm/compiler_stats.cc
+++ b/runtime/vm/compiler_stats.cc
@@ -123,9 +123,13 @@ void CompilerStats::Update() {
   // Traverse the heap and compute number of tokens in all
   // TokenStream objects.
   num_tokens_total = 0;
-  TokenStreamVisitor visitor(this);
-  isolate_->heap()->IterateObjects(&visitor);
-  Dart::vm_isolate()->heap()->IterateObjects(&visitor);
+
+  {
+    HeapIterationScope iteration;
+    TokenStreamVisitor visitor(this);
+    iteration.IterateObjects(&visitor);
+    iteration.IterateVMIsolateObjects(&visitor);
+  }
 }
 
 void CompilerStats::EnableBenchmark() {