[es6] Partially implement Reflect.preventExtensions.

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index ae2467526352512d96004d47f2caf0222865113d..202d9af1b975d7c8692d17dfe6b07f448d39453a 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -698,7 +698,8 @@ Handle<JSFunction> Genesis::GetThrowTypeErrorIntrinsic(
       static_cast<PropertyAttributes>(DONT_ENUM | DONT_DELETE | READ_ONLY))
       .Assert();
 
-  JSObject::PreventExtensions(function).Assert();
+  if (JSObject::PreventExtensions(function, THROW_ON_ERROR).IsNothing())
+    DCHECK(false);
 
   return function;
 }
@@ -1925,6 +1926,8 @@ void Genesis::InitializeGlobal_harmony_reflect() {
                         Builtins::kReflectHas, 2, true);
   SimpleInstallFunction(reflect, "isExtensible",
                         Builtins::kReflectIsExtensible, 1, true);
+  SimpleInstallFunction(reflect, "preventExtensions",
+                        Builtins::kReflectPreventExtensions, 1, true);
 }