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

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index 4bc1125f23058b732b486cee122f850f03dd89db..33ed0638759e34ba384a91dc60a14b15aee78be3 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -1500,8 +1500,11 @@ Dart_CreateSnapshot(uint8_t** vm_snapshot_data_buffer,
 
 #if defined(DEBUG)
   I->heap()->CollectAllGarbage();
-  CheckFunctionTypesVisitor check_canonical(T);
-  I->heap()->IterateObjects(&check_canonical);
+  {
+    HeapIterationScope iteration(T);
+    CheckFunctionTypesVisitor check_canonical(T);
+    iteration.IterateObjects(&check_canonical);
+  }
 #endif  // #if defined(DEBUG)
 
   Symbols::Compact(I);
@@ -1535,8 +1538,11 @@ Dart_CreateScriptSnapshot(uint8_t** script_snapshot_buffer,
 
 #if defined(DEBUG)
   I->heap()->CollectAllGarbage();
-  CheckFunctionTypesVisitor check_canonical(T);
-  I->heap()->IterateObjects(&check_canonical);
+  {
+    HeapIterationScope iteration(T);
+    CheckFunctionTypesVisitor check_canonical(T);
+    iteration.IterateObjects(&check_canonical);
+  }
 #endif  // #if defined(DEBUG)
 
   ScriptSnapshotWriter writer(script_snapshot_buffer, ApiReallocate);