Add mutable double boxes for fields.

runtime/vm/flow_graph_builder.cc

Index: runtime/vm/flow_graph_builder.cc
===================================================================
--- runtime/vm/flow_graph_builder.cc	(revision 31125)
+++ runtime/vm/flow_graph_builder.cc	(working copy)
@@ -3285,18 +3285,12 @@
       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());