Use mutable heapnumbers to store doubles in fields.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index d83d29108fcacf2106b085790d59faeb32e0e1b0..cb67e367ed61e3f071033fd5e13919d3bd8b0e7f 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1080,6 +1080,8 @@ class Object : public MaybeObject {
     return true;
   }
 
+  inline MaybeObject* StorageFor(Heap* heap, Representation representation);
+
   // Returns true if the object is of the correct type to be used as a
   // implementation of a JSObject's elements.
   inline bool HasValidElements();
@@ -2135,10 +2137,12 @@ class JSObject: public JSReceiver {
 
   // Add a property to a fast-case object using a map transition to
   // new_map.
-  MUST_USE_RESULT MaybeObject* AddFastPropertyUsingMap(Map* new_map,
-                                                       Name* name,
-                                                       Object* value,
-                                                       int field_index);
+  MUST_USE_RESULT MaybeObject* AddFastPropertyUsingMap(
+      Map* new_map,
+      Name* name,
+      Object* value,
+      int field_index,
+      Representation representation);
 
   // Add a constant function property to a fast-case object.
   // This leaves a CONSTANT_TRANSITION in the old map, and
@@ -2248,7 +2252,7 @@ class JSObject: public JSReceiver {
 
   // Access fast-case object properties at index.
   inline Object* FastPropertyAt(int index);
-  inline Object* FastPropertyAtPut(int index, Object* value);
+  inline void FastPropertyAtPut(int index, Object* value);
 
   // Access to in object properties.
   inline int GetInObjectPropertyOffset(int index);
@@ -5199,7 +5203,8 @@ class Map: public HeapObject {
 
   int NumberOfFields();
 
-  bool InstancesNeedRewriting(int target_number_of_fields,
+  bool InstancesNeedRewriting(Map* target,
+                              int target_number_of_fields,
                               int target_inobject,
                               int target_unused);
   static Handle<Map> GeneralizeRepresentation(