Use null instead of undefined for deleted elements in code caches....

src/objects-inl.h

Index: src/objects-inl.h
===================================================================
--- src/objects-inl.h	(revision 338)
+++ src/objects-inl.h	(working copy)
@@ -977,6 +977,13 @@
 }
 
 
+void FixedArray::set_null(int index) {
+  ASSERT(index >= 0 && index < this->length());
+  ASSERT(!Heap::InNewSpace(Heap::null_value()));
+  WRITE_FIELD(this, kHeaderSize + index * kPointerSize, Heap::null_value());
+}
+
+
 void FixedArray::set_the_hole(int index) {
   ASSERT(index >= 0 && index < this->length());
   ASSERT(!Heap::InNewSpace(Heap::the_hole_value()));