runtime: test that Symbol.toStringTag not set when already defined

@ExE-Boss I (@benjamn) modified your runtime changes from #399 in favor of

Author: ExE-Boss

Diff for: test/non-writable-tostringtag-property.js

@@ -0,0 +1,27 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+var hasOwn = Object.prototype.hasOwnProperty;
+var getProto = Object.getPrototypeOf;
+var NativeIteratorPrototype =
+  typeof Symbol === "function" &&
+  typeof Symbol.iterator === "symbol" &&
+  typeof [][Symbol.iterator] === "function" &&
+  getProto &&
+  getProto(getProto([][Symbol.iterator]()));
+
+// https://tc39.es/proposal-iterator-helpers/#sec-iteratorprototype-@@tostringtag
+Object.defineProperty(NativeIteratorPrototype, Symbol.toStringTag, {
+  configurable: true,
+  value: "Iterator",
+});
+
+describe("Symbol.toStringTag safety (#399, #400)", function () {
+  it("regenerator-runtime doesn't fail to initialize when native iterator prototype has a non-writable @@toStringTag property", function() {
+    require("./runtime.js");
+  });
+});

Diff for: test/run.js

@@ -133,6 +133,14 @@ if (semver.gte(process.version, "4.0.0")) {
   ]);
 }
 
+if (semver.gte(process.version, "4.0.0")) {
+  enqueue("mocha", [
+    "--harmony",
+    "--reporter", "spec",
+    "./test/non-writable-tostringtag-property.js",
+  ]);
+}
+
 enqueue(convert, [
   "./test/tests.es6.js",
   "./test/tests.es5.js"