Create and cache method extraction stub in the ICData.

runtime/vm/flow_graph.cc

Index: runtime/vm/flow_graph.cc
diff --git a/runtime/vm/flow_graph.cc b/runtime/vm/flow_graph.cc
index 0ead0a280ec731cea8804166a4277d14c6031487..d57a8ec1b49a67faf687a18c4f802d1ac386cf4a 100644
--- a/runtime/vm/flow_graph.cc
+++ b/runtime/vm/flow_graph.cc
@@ -508,7 +508,7 @@ void FlowGraph::Rename(GrowableArray<PhiInstr*>* live_phis,
   GrowableArray<Definition*> env(variable_count());
 
   // Add global constants to the initial definitions.
-  ConstantInstr* constant_null =
+  constant_null_ =
       AddConstantToInitialDefinitions(Object::ZoneHandle());
 
   // Add parameters to the initial definitions and renaming environment.
@@ -533,7 +533,7 @@ void FlowGraph::Rename(GrowableArray<PhiInstr*>* live_phis,
 
   // Initialize all locals with #null in the renaming environment.
   for (intptr_t i = parameter_count(); i < variable_count(); ++i) {
-    env.Add(constant_null);
+    env.Add(constant_null());
   }
 
   BlockEntryInstr* normal_entry = graph_entry_->SuccessorAt(0);