Validate well-formedness of the use lists in debug mode.

runtime/vm/flow_graph.h

Index: runtime/vm/flow_graph.h
diff --git a/runtime/vm/flow_graph.h b/runtime/vm/flow_graph.h
index 710d0583ba06ff6e7615d6aae85352f056aaf221..3d7c51d12e2f90d7b0c58fd43ef695a2fe34d4cb 100644
--- a/runtime/vm/flow_graph.h
+++ b/runtime/vm/flow_graph.h
@@ -69,6 +69,12 @@ class FlowGraph: public ZoneAllocated {
   // TODO(zerny): Once the SSA is feature complete this should be removed.
   void Bailout(const char* reason) const;
 
+#ifdef DEBUG
+  // Validation methods for debugging.
+  bool ResetUseLists();
+  bool ValidateUseLists();
+#endif  // DEBUG
+
  private:
   void DiscoverBlocks();