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

src/compiler/typer.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/compiler/typer.h"
 
 #include "src/base/flags.h"
 #include "src/bootstrapper.h"
 #include "src/compilation-dependencies.h"
 #include "src/compiler/common-operator.h"
@@ skipping 1398 matching lines @@
 
 Type* Typer::Visitor::TypeJSInstanceOf(Node* node) {
   return Type::Boolean(zone());
 }
 
 
 // JS context operators.
 
 
 Type* Typer::Visitor::TypeJSLoadContext(Node* node) {
+  ContextAccess const& access = ContextAccessOf(node->op());
+  if (access.index() == Context::EXTENSION_INDEX) {
+    return Type::TaggedPointer();
+  }
   // Since contexts are mutable, we just return the top.
   return Type::Any();
 }
 
 
 Type* Typer::Visitor::TypeJSStoreContext(Node* node) {
   UNREACHABLE();
   return nullptr;
 }
 
@@ skipping 984 matching lines @@
   }
   if (Type::IsInteger(*value)) {
     return Type::Range(value->Number(), value->Number(), zone());
   }
   return Type::Constant(value, zone());
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8