Make generator constructors configurable

test/mjsunit/es6/regress/regress-3902.js

+// Copyright 2015 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.
+
+function* g() {}
+assertTrue(Object.getOwnPropertyDescriptor(g.__proto__, "constructor").configurable);
+assertTrue(Object.getOwnPropertyDescriptor(g.prototype.__proto__, "constructor").configurable);
+
+function FakeGeneratorFunctionConstructor() {}
+Object.defineProperty(g.__proto__, "constructor", {value: FakeGeneratorFunctionConstructor});
+assertSame(g.__proto__.constructor, FakeGeneratorFunctionConstructor);
+
+function FakeGeneratorObjectConstructor() {}
+Object.defineProperty(g.prototype.__proto__, "constructor", {value: FakeGeneratorObjectConstructor});
+assertSame(g.prototype.__proto__.constructor, FakeGeneratorObjectConstructor);