[es6] Define generator prototype as writable prop

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/utils/random-number-generator.h"
 #include "src/code-stubs.h"
@@ skipping 2159 matching lines @@
         factory()->InternalizeUtf8String("prototype"),
         generator_object_prototype,
         static_cast<PropertyAttributes>(DONT_ENUM | READ_ONLY));
 
     static const bool kUseStrictFunctionMap = true;
     InstallFunction(builtins, "GeneratorFunction", JS_FUNCTION_TYPE,
                     JSFunction::kSize, generator_function_prototype,
                     Builtins::kIllegal, kUseStrictFunctionMap);
 
     // Create maps for generator functions and their prototypes.  Store those
-    // maps in the native context. Generator functions do not have writable
-    // prototypes, nor do they have "caller" or "arguments" accessors.
-    Handle<Map> strict_function_map(native_context()->strict_function_map());
+    // maps in the native context. The "prototype" property descriptor is
+    // writable, non-enumerable, and non-configurable (as per ES6 draft
+    // 04-14-15, section 25.2.4.3).
+    Handle<Map> strict_function_map(strict_function_map_writable_prototype_);
+    // Generator functions do not have "caller" or "arguments" accessors.
     Handle<Map> sloppy_generator_function_map =
         Map::Copy(strict_function_map, "SloppyGeneratorFunction");
     Map::SetPrototype(sloppy_generator_function_map,
                       generator_function_prototype);
     native_context()->set_sloppy_generator_function_map(
         *sloppy_generator_function_map);
 
     Handle<Map> strict_generator_function_map =
         Map::Copy(strict_function_map, "StrictGeneratorFunction");
     Map::SetPrototype(strict_generator_function_map,
@@ skipping 852 matching lines @@
   return from + sizeof(NestingCounterType);
 }
 
 
 // Called when the top-level V8 mutex is destroyed.
 void Bootstrapper::FreeThreadResources() {
   DCHECK(!IsActive());
 }
 
 } }  // namespace v8::internal