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

src/code-stubs-hydrogen.cc

 // 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.
 
 #include "src/code-stubs.h"
 
 #include "src/bailout-reason.h"
 #include "src/crankshaft/hydrogen.h"
 #include "src/crankshaft/lithium.h"
 #include "src/field-index.h"
@@ skipping 2117 matching lines @@
 
   // Set up the fixed slots.
   Add<HStoreNamedField>(function_context,
                         HObjectAccess::ForContextSlot(Context::CLOSURE_INDEX),
                         function);
   Add<HStoreNamedField>(function_context,
                         HObjectAccess::ForContextSlot(Context::PREVIOUS_INDEX),
                         context());
   Add<HStoreNamedField>(function_context,
                         HObjectAccess::ForContextSlot(Context::EXTENSION_INDEX),
-                        graph()->GetConstant0());
+                        graph()->GetConstantHole());
 
   // Copy the native context from the previous context.
   HValue* native_context = Add<HLoadNamedField>(
       context(), nullptr,
       HObjectAccess::ForContextSlot(Context::NATIVE_CONTEXT_INDEX));
   Add<HStoreNamedField>(function_context, HObjectAccess::ForContextSlot(
                                               Context::NATIVE_CONTEXT_INDEX),
                         native_context);
 
   // Initialize the rest of the slots to undefined.
@@ skipping 295 matching lines @@
   return Pop();
 }
 
 
 Handle<Code> KeyedLoadGenericStub::GenerateCode() {
   return DoGenerateCode(this);
 }
 
 }  // namespace internal
 }  // namespace v8