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

runtime/vm/clustered_snapshot.cc

Index: runtime/vm/clustered_snapshot.cc
diff --git a/runtime/vm/clustered_snapshot.cc b/runtime/vm/clustered_snapshot.cc
index 6bc3bfe896f83e310fee6d1abebcede823b2c5dd..7948c6d39b3847e1177a6acc4a50c6333cdc902f 100644
--- a/runtime/vm/clustered_snapshot.cc
+++ b/runtime/vm/clustered_snapshot.cc
@@ -5349,15 +5349,18 @@ FullSnapshotWriter::FullSnapshotWriter(Snapshot::Kind kind,
     // it out as part of the VM isolate snapshot. We first count the number of
     // token streams, allocate an array and then fill it up with the token
     // streams.
-    SnapshotTokenStreamVisitor token_streams_counter(thread());
-    heap()->IterateOldObjects(&token_streams_counter);
-    Dart::vm_isolate()->heap()->IterateOldObjects(&token_streams_counter);
-    intptr_t count = token_streams_counter.count();
-    token_streams_ = Array::New(count, Heap::kOld);
-    SnapshotTokenStreamVisitor script_visitor(thread(), &token_streams_);
-    heap()->IterateOldObjects(&script_visitor);
-    Dart::vm_isolate()->heap()->IterateOldObjects(&script_visitor);
-    ASSERT(script_visitor.count() == count);
+    {
+      HeapIterationScope iteration(thread());
+      SnapshotTokenStreamVisitor token_streams_counter(thread());
+      iteration.IterateObjects(&token_streams_counter);
+      iteration.IterateVMIsolateObjects(&token_streams_counter);
+      intptr_t count = token_streams_counter.count();
+      token_streams_ = Array::New(count, Heap::kOld);
+      SnapshotTokenStreamVisitor script_visitor(thread(), &token_streams_);
+      iteration.IterateObjects(&script_visitor);
+      iteration.IterateVMIsolateObjects(&script_visitor);
+      ASSERT(script_visitor.count() == count);
+    }
 
     // Tuck away the current symbol table.
     saved_symbol_table_ = object_store->symbol_table();