Create and cache method extraction stub in the ICData.

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 fd3709c9529ce1d52c7784a1ffecbddc04e14727..67579887102b6e6e77441dc8ed516e9737a8c660 100644
--- a/runtime/vm/flow_graph.h
+++ b/runtime/vm/flow_graph.h
@@ -98,6 +98,10 @@ class FlowGraph : public ZoneAllocated {
     return graph_entry_;
   }
 
+  ConstantInstr* constant_null() const {
+    return constant_null_;
+  }
+
   ZoneGrowableArray<ReturnInstr*>* exits() const { return exits_; }
   void set_exits(ZoneGrowableArray<ReturnInstr*>* exits) { exits_ = exits; }
 
@@ -179,6 +183,7 @@ class FlowGraph : public ZoneAllocated {
   GrowableArray<BlockEntryInstr*> reverse_postorder_;
   ZoneGrowableArray<ReturnInstr*>* exits_;
   bool invalid_dominator_tree_;
+  ConstantInstr* constant_null_;
 };
 
 }  // namespace dart