Use mutable heapnumbers to store doubles in fields.

src/property.h

Index: src/property.h
diff --git a/src/property.h b/src/property.h
index 229241948a8c4ee0dc253e199bdbd1710fdb8159..7a83f9db789d6e63cbcd15529380bd4c14bf2d28 100644
--- a/src/property.h
+++ b/src/property.h
@@ -268,6 +268,8 @@ class LookupResult BASE_EMBEDDED {
 
   Representation representation() {
     ASSERT(IsFound());
+    ASSERT(!IsTransition());
+    ASSERT(details_.type() != NONEXISTENT);
     return details_.representation();
   }
 
@@ -355,7 +357,7 @@ class LookupResult BASE_EMBEDDED {
   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());