Use mutable heapnumbers to store doubles in fields.

src/property.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 250 matching lines @@
     return JSProxy::cast(holder_);
   }
 
   PropertyType type() {
     ASSERT(IsFound());
     return details_.type();
   }
 
   Representation representation() {
     ASSERT(IsFound());
+    ASSERT(!IsTransition());
+    ASSERT(details_.type() != NONEXISTENT);
     return details_.representation();
   }
 
   PropertyAttributes GetAttributes() {
     ASSERT(!IsTransition());
     ASSERT(IsFound());
     ASSERT(details_.type() != NONEXISTENT);
     return details_.attributes();
   }
 
@@ skipping 67 matching lines @@
     UNREACHABLE();
     return false;
   }
 
   bool IsCacheable() { return cacheable_; }
   void DisallowCaching() { cacheable_ = false; }
 
   Object* GetLazyValue() {
     switch (type()) {
       case FIELD:
-        return holder()->FastPropertyAt(GetFieldIndex().field_index());
+        return holder()->RawFastPropertyAt(GetFieldIndex().field_index());
       case NORMAL: {
         Object* value;
         value = holder()->property_dictionary()->ValueAt(GetDictionaryEntry());
         if (holder()->IsGlobalObject()) {
           value = JSGlobalPropertyCell::cast(value)->value();
         }
         return value;
       }
       case CONSTANT_FUNCTION:
         return GetConstantFunction();
@@ skipping 123 matching lines @@
   JSReceiver* holder_;
   int number_;
   bool cacheable_;
   PropertyDetails details_;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_PROPERTY_H_