Make sure builtins preserve guarantees about empty element array prototypes.

src/hydrogen.cc

 // Copyright 2013 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/hydrogen.h"
 
 #include <sstream>
 
 #include "src/v8.h"
 
@@ skipping 8433 matching lines @@
       // inlined version can't be used.
       if (receiver_map->DictionaryElementsInPrototypeChainOnly()) return false;
       // If there currently can be no elements accessors on the prototype chain,
       // it doesn't mean that there won't be any later. Install a full prototype
       // chain check to trap element accessors being installed on the prototype
       // chain, which would cause elements to go to dictionary mode and result
       // in a map change.
       Handle<JSObject> prototype(JSObject::cast(receiver_map->prototype()));
       BuildCheckPrototypeMaps(prototype, Handle<JSObject>());
 
+      // Protect against adding elements to the Array prototype, which needs to
+      // route through appropriate bottlenecks.
+      if (isolate()->IsFastArrayConstructorPrototypeChainIntact() &&
+          !prototype->IsJSArray()) {
+        return false;
+      }
+
       const int argc = args_count_no_receiver;
       if (argc != 1) return false;
 
       HValue* value_to_push = Pop();
       HValue* array = Pop();
       Drop(1);  // Drop function.
 
       HInstruction* new_size = NULL;
       HValue* length = NULL;
 
@@ skipping 4501 matching lines @@
   if (ShouldProduceTraceOutput()) {
     isolate()->GetHTracer()->TraceHydrogen(name(), graph_);
   }
 
 #ifdef DEBUG
   graph_->Verify(false);  // No full verify.
 #endif
 }
 
 } }  // namespace v8::internal