[runtime] Use "the hole" instead of smi 0 as sentinel for context extension.

src/contexts.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_CONTEXTS_H_
 #define V8_CONTEXTS_H_
 
 #include "src/heap/heap.h"
 #include "src/objects.h"
 
@@ skipping 309 matching lines @@
 //                or may not become the current function's context), and
 //                it provides access to the functions code and thus it's
 //                scope information, which in turn contains the names of
 //                statically allocated context slots. The names are needed
 //                for dynamic lookups in the presence of 'with' or 'eval'.
 //
 // [ previous  ]  A pointer to the previous context. It is NULL for
 //                function contexts, and non-NULL for 'with' contexts.
 //                Used to implement the 'with' statement.
 //
-// [ extension ]  A pointer to an extension JSObject, or NULL. Used to
+// [ extension ]  A pointer to an extension JSObject, or "the hole". Used to
 //                implement 'with' statements and dynamic declarations
 //                (through 'eval'). The object in a 'with' statement is
 //                stored in the extension slot of a 'with' context.
 //                Dynamically declared variables/functions are also added
 //                to lazily allocated extension object. Context::Lookup
 //                searches the extension object for properties.
 //                For script and block contexts, contains the respective
 //                ScopeInfo. For block contexts representing sloppy declaration
 //                block scopes, it may also be a struct being a
 //                SloppyBlockWithEvalContextExtension, pairing the ScopeInfo
@@ skipping 57 matching lines @@
   int GetErrorsThrown();
 
   // Direct slot access.
   inline JSFunction* closure();
   inline void set_closure(JSFunction* closure);
 
   inline Context* previous();
   inline void set_previous(Context* context);
 
   inline bool has_extension();
-  inline Object* extension();
-  inline void set_extension(Object* object);
+  inline HeapObject* extension();
+  inline void set_extension(HeapObject* object);
   JSObject* extension_object();
   JSReceiver* extension_receiver();
   ScopeInfo* scope_info();
   String* catch_name();
 
   inline JSModule* module();
   inline void set_module(JSModule* module);
 
   // Get the context where var declarations will be hoisted to, which
   // may be the context itself.
@@ skipping 132 matching lines @@
 #endif
 
   STATIC_ASSERT(kHeaderSize == Internals::kContextHeaderSize);
   STATIC_ASSERT(EMBEDDER_DATA_INDEX == Internals::kContextEmbedderDataIndex);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_CONTEXTS_H_