Add mutable double boxes for fields.

runtime/vm/flow_graph_builder.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/flow_graph_builder.h"
 
 #include "lib/invocation_mirror.h"
 #include "vm/ast_printer.h"
 #include "vm/bit_vector.h"
 #include "vm/class_finalizer.h"
@@ skipping 3267 matching lines @@
       new GuardFieldInstr(store_value,
                           node->field(),
                           Isolate::Current()->GetNextDeoptId());
   AddInstruction(guard);
 
   store_value = Bind(BuildLoadExprTemp());
   StoreInstanceFieldInstr* store =
       new StoreInstanceFieldInstr(node->field(),
                                   for_instance.value(),
                                   store_value,
-                                  kEmitStoreBarrier);
+                                  kEmitStoreBarrier,
+                                  true);  // Maybe initializing store.
   ReturnDefinition(store);
 }
 
 
-// StoreInstanceFieldNode does not return result.
-void ValueGraphVisitor::VisitStoreInstanceFieldNode(
-    StoreInstanceFieldNode* node) {
-  UNIMPLEMENTED();
-}
-
-
 void EffectGraphVisitor::VisitLoadStaticFieldNode(LoadStaticFieldNode* node) {
   if (node->field().is_const()) {
     ASSERT(node->field().value() != Object::sentinel().raw());
     ASSERT(node->field().value() != Object::transition_sentinel().raw());
     Definition* result =
         new ConstantInstr(Instance::ZoneHandle(node->field().value()));
     return ReturnDefinition(result);
   }
   Value* field_value = Bind(new ConstantInstr(node->field()));
   LoadStaticFieldInstr* load = new LoadStaticFieldInstr(field_value);
@@ skipping 732 matching lines @@
                                   LanguageError::kError,
                                   Heap::kNew,
                                   "FlowGraphBuilder Bailout: %s %s",
                                   String::Handle(function.name()).ToCString(),
                                   reason));
   Isolate::Current()->long_jump_base()->Jump(1, error);
 }
 
 
 }  // namespace dart