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 c3cffb9229536e578f92b87f6badbe35c3dc7ae6..ebd57af232dc63ae23b6a540fca33ffd7fd43848 100644
--- a/runtime/vm/flow_graph.h
+++ b/runtime/vm/flow_graph.h
@@ -99,6 +99,10 @@ class FlowGraph : public ZoneAllocated {
     return graph_entry_;
   }
 
+  ConstantInstr* constant_null() const {
+    return constant_null_;
+  }
+
   intptr_t alloc_ssa_temp_index() { return current_ssa_temp_index_++; }
 
   intptr_t InstructionCount() const;
@@ -178,6 +182,7 @@ class FlowGraph : public ZoneAllocated {
   GrowableArray<BlockEntryInstr*> postorder_;
   GrowableArray<BlockEntryInstr*> reverse_postorder_;
   bool invalid_dominator_tree_;
+  ConstantInstr* constant_null_;
 };
 
 }  // namespace dart