[regexp] Cache the correct initial prototype map

src/bootstrapper.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/bootstrapper.h"
 
 #include "src/accessors.h"
 #include "src/api-natives.h"
 #include "src/base/ieee754.h"
 #include "src/code-stubs.h"
@@ skipping 1864 matching lines @@
         InstallFunction(prototype, fun, factory->search_symbol(), DONT_ENUM);
       }
 
       {
         Handle<JSFunction> fun = SimpleCreateFunction(
             isolate, factory->InternalizeUtf8String("[Symbol.split]"),
             Builtins::kRegExpPrototypeSplit, 2, false);
         InstallFunction(prototype, fun, factory->split_symbol(), DONT_ENUM);
       }
 
+      Handle<Map> prototype_map(prototype->map());
+      Map::SetShouldBeFastPrototypeMap(prototype_map, true, isolate);
+
       // Store the initial RegExp.prototype map. This is used in fast-path
       // checks. Do not alter the prototype after this point.
-      native_context()->set_regexp_prototype_map(prototype->map());
+      native_context()->set_regexp_prototype_map(*prototype_map);
     }
 
     {
       // RegExp getters and setters.
 
       SimpleInstallGetter(regexp_fun,
                           factory->InternalizeUtf8String("[Symbol.species]"),
                           factory->species_symbol(),
                           Builtins::kRegExpPrototypeSpeciesGetter, false);
 
@@ skipping 2660 matching lines @@
 }
 
 
 // Called when the top-level V8 mutex is destroyed.
 void Bootstrapper::FreeThreadResources() {
   DCHECK(!IsActive());
 }
 
 }  // namespace internal
 }  // namespace v8