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

src/contexts.cc

 // Copyright 2011 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/contexts.h"
 
 #include "src/ast/scopeinfo.h"
 #include "src/bootstrapper.h"
 #include "src/debug/debug.h"
 #include "src/isolate-inl.h"
@@ skipping 64 matching lines @@
   while (!current->is_declaration_context()) {
     current = current->previous();
     DCHECK(current->closure() == closure());
   }
   return current;
 }
 
 
 JSObject* Context::extension_object() {
   DCHECK(IsNativeContext() || IsFunctionContext() || IsBlockContext());
-  Object* object = extension();
-  if (object == nullptr) return nullptr;
+  HeapObject* object = extension();
+  if (object->IsTheHole()) return nullptr;
   if (IsBlockContext()) {
     if (!object->IsSloppyBlockWithEvalContextExtension()) return nullptr;
     object = SloppyBlockWithEvalContextExtension::cast(object)->extension();
   }
   DCHECK(object->IsJSContextExtensionObject() ||
          (IsNativeContext() && object->IsJSGlobalObject()));
   return JSObject::cast(object);
 }
 
 
 JSReceiver* Context::extension_receiver() {
   DCHECK(IsNativeContext() || IsWithContext() ||
          IsFunctionContext() || IsBlockContext());
   return IsWithContext() ? JSReceiver::cast(extension()) : extension_object();
 }
 
 
 ScopeInfo* Context::scope_info() {
   DCHECK(IsModuleContext() || IsScriptContext() || IsBlockContext());
-  Object* object = extension();
+  HeapObject* object = extension();
   if (object->IsSloppyBlockWithEvalContextExtension()) {
     DCHECK(IsBlockContext());
     object = SloppyBlockWithEvalContextExtension::cast(object)->scope_info();
   }
   return ScopeInfo::cast(object);
 }
 
 
 String* Context::catch_name() {
   DCHECK(IsCatchContext());
@@ skipping 462 matching lines @@
 
   int previous_value = errors_thrown()->value();
   set_errors_thrown(Smi::FromInt(previous_value + 1));
 }
 
 
 int Context::GetErrorsThrown() { return errors_thrown()->value(); }
 
 }  // namespace internal
 }  // namespace v8