Give each function prototype a fresh copy of the object function map.

src/heap.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 3239 matching lines @@
   return function;
 }
 
 
 MaybeObject* Heap::AllocateFunctionPrototype(JSFunction* function) {
   // Allocate the prototype.  Make sure to use the object function
   // from the function's context, since the function can be from a
   // different context.
   JSFunction* object_function =
       function->context()->global_context()->object_function();
+
+  // Each function prototype gets a copy of the object function map.
+  // This avoid unwanted sharing of maps between prototypes of different
+  // constructors.
+  Map* new_map;
+  ASSERT(object_function->has_initial_map());
+  { MaybeObject* maybe_map =
+        object_function->initial_map()->CopyDropTransitions();
+    if (!maybe_map->To<Map>(&new_map)) return maybe_map;
+  }
   Object* prototype;
-  { MaybeObject* maybe_prototype = AllocateJSObject(object_function);
+  { MaybeObject* maybe_prototype = AllocateJSObjectFromMap(new_map);
     if (!maybe_prototype->ToObject(&prototype)) return maybe_prototype;
   }
   // When creating the prototype for the function we must set its
   // constructor to the function.
   Object* result;
   { MaybeObject* maybe_result =
         JSObject::cast(prototype)->SetLocalPropertyIgnoreAttributes(
             constructor_symbol(), function, DONT_ENUM);
     if (!maybe_result->ToObject(&result)) return maybe_result;
   }
@@ skipping 3084 matching lines @@
   }
   isolate_->heap()->store_buffer()->Filter(MemoryChunk::ABOUT_TO_BE_FREED);
   for (chunk = chunks_queued_for_free_; chunk != NULL; chunk = next) {
     next = chunk->next_chunk();
     isolate_->memory_allocator()->Free(chunk);
   }
   chunks_queued_for_free_ = NULL;
 }
 
 } }  // namespace v8::internal