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

src/runtime/runtime-scopes.cc

 // Copyright 2014 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.
 
 #include "src/runtime/runtime-utils.h"
 
 #include "src/accessors.h"
 #include "src/arguments.h"
 #include "src/ast/scopeinfo.h"
 #include "src/ast/scopes.h"
@@ skipping 276 matching lines @@
 
     object = Handle<JSObject>::cast(holder);
 
   } else if (context->has_extension()) {
     // Sloppy varblock contexts might not have an extension object yet,
     // in which case their extension is a ScopeInfo.
     if (context->extension()->IsScopeInfo()) {
       DCHECK(context->IsBlockContext());
       object = isolate->factory()->NewJSObject(
           isolate->context_extension_function());
-      Handle<Object> extension =
+      Handle<HeapObject> extension =
           isolate->factory()->NewSloppyBlockWithEvalContextExtension(
               handle(context->scope_info()), object);
       context->set_extension(*extension);
     } else {
       object = handle(context->extension_object(), isolate);
     }
     DCHECK(object->IsJSContextExtensionObject() || object->IsJSGlobalObject());
   } else {
     DCHECK(context->IsFunctionContext());
     object =
@@ skipping 820 matching lines @@
 
   // Lookup in the initial Object.prototype object.
   Handle<Object> result;
   ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
       isolate, result,
       Object::GetProperty(isolate->initial_object_prototype(), key));
   return *result;
 }
 }  // namespace internal
 }  // namespace v8