Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index a494778a989c3c3963f8ba288b18ec5fa5359b58..94e974ca504942c1901e575e275483b26e8a4dea 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -168,6 +168,12 @@ enum ContextualMode {
 };
 
 
+enum MutableMode {
+  MUTABLE,
+  IMMUTABLE
+};
+
+
 static const int kGrowICDelta = STORE_AND_GROW_NO_TRANSITION -
     STANDARD_STORE;
 STATIC_ASSERT(STANDARD_STORE == 0);
@@ -352,6 +358,7 @@ const int kStubMinorKeyBits = kBitsPerInt - kSmiTagSize - kStubMajorKeyBits;
   V(PROPERTY_CELL_TYPE)                                                        \
                                                                                \
   V(HEAP_NUMBER_TYPE)                                                          \
+  V(MUTABLE_HEAP_NUMBER_TYPE)                                                  \
   V(FOREIGN_TYPE)                                                              \
   V(BYTE_ARRAY_TYPE)                                                           \
   V(FREE_SPACE_TYPE)                                                           \
@@ -680,6 +687,7 @@ enum InstanceType {
   // "Data", objects that cannot contain non-map-word pointers to heap
   // objects.
   HEAP_NUMBER_TYPE,
+  MUTABLE_HEAP_NUMBER_TYPE,
   FOREIGN_TYPE,
   BYTE_ARRAY_TYPE,
   FREE_SPACE_TYPE,
@@ -900,6 +908,7 @@ template <class C> inline bool Is(Object* obj);
 
 #define HEAP_OBJECT_TYPE_LIST(V)               \
   V(HeapNumber)                                \
+  V(MutableHeapNumber)                         \
   V(Name)                                      \
   V(UniqueName)                                \
   V(String)                                    \
@@ -1427,7 +1436,7 @@ class Object {
     } else if (FLAG_track_fields && representation.IsSmi()) {
       return IsSmi();
     } else if (FLAG_track_double_fields && representation.IsDouble()) {
-      return IsNumber();
+      return IsMutableHeapNumber() || IsNumber();
     } else if (FLAG_track_heap_object_fields && representation.IsHeapObject()) {
       return IsHeapObject();
     }
@@ -1440,6 +1449,10 @@ class Object {
                                              Handle<Object> object,
                                              Representation representation);
 
+  inline static Handle<Object> WrapForRead(Isolate* isolate,
+                                           Handle<Object> object,
+                                           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();