Replace ChainContextInstr with existing IL instructions.

runtime/vm/flow_graph_optimizer.cc

Index: runtime/vm/flow_graph_optimizer.cc
===================================================================
--- runtime/vm/flow_graph_optimizer.cc	(revision 25088)
+++ runtime/vm/flow_graph_optimizer.cc	(working copy)
@@ -3791,7 +3791,6 @@
         *is_load = true;
         break;
 
-      case Instruction::kChainContext:
       case Instruction::kStoreContext:
         kind_ = kContext;
         break;
@@ -4017,7 +4016,7 @@
       return Alias::VMField(store_vm_field->offset_in_bytes());
     }
 
-    if (instr->IsStoreContext() || instr->IsChainContext()) {
+    if (instr->IsStoreContext()) {
       return Alias::CurrentContext();
     }
 
@@ -4259,7 +4258,7 @@
     return store_static_field->value()->definition();
   }
 
-  if (instr->IsStoreContext() || instr->IsChainContext()) {
+  if (instr->IsStoreContext()) {
     return instr->InputAt(0)->definition();
   }
 
@@ -5510,9 +5509,6 @@
 void ConstantPropagator::VisitStoreContext(StoreContextInstr* instr) { }
 
 
-void ConstantPropagator::VisitChainContext(ChainContextInstr* instr) { }
-
-
 void ConstantPropagator::VisitCatchEntry(CatchEntryInstr* instr) { }