Fix generator prototype chain to more correctly match ES2015 spec

packages/regenerator-runtime/runtime.js

@@ -11,7 +11,8 @@
 !(function(global) {
   "use strict";
 
-  var hasOwn = Object.prototype.hasOwnProperty;
+  var Op = Object.prototype;
+  var hasOwn = Op.hasOwnProperty;
   var undefined; // More compressible than void 0.
   var $Symbol = typeof Symbol === "function" ? Symbol : {};
   var iteratorSymbol = $Symbol.iterator || "@@iterator";
@@ -83,10 +84,29 @@
   function GeneratorFunction() {}
   function GeneratorFunctionPrototype() {}
 
-  var Gp = GeneratorFunctionPrototype.prototype = Generator.prototype;
+  // This is a polyfill for %IteratorPrototype% for environments that
+  // don't natively support it.
+  var IteratorPrototype = {};
+  IteratorPrototype[iteratorSymbol] = function () {
+    return this;
+  };
+
+  var getProto = Object.getPrototypeOf;
+  var NativeIteratorPrototype = getProto && getProto(getProto(values([])));
+  if (NativeIteratorPrototype &&
+      NativeIteratorPrototype !== Op &&
+      hasOwn.call(NativeIteratorPrototype, iteratorSymbol)) {
+    // This environment has a native %IteratorPrototype%; use it instead
+    // of the polyfill.
+    IteratorPrototype = NativeIteratorPrototype;
+  }
+
+  var Gp = GeneratorFunctionPrototype.prototype =
+    Generator.prototype = Object.create(IteratorPrototype);
   GeneratorFunction.prototype = Gp.constructor = GeneratorFunctionPrototype;
   GeneratorFunctionPrototype.constructor = GeneratorFunction;
-  GeneratorFunctionPrototype[toStringTagSymbol] = GeneratorFunction.displayName = "GeneratorFunction";
+  GeneratorFunctionPrototype[toStringTagSymbol] =
+    GeneratorFunction.displayName = "GeneratorFunction";
 
   // Helper for defining the .next, .throw, and .return methods of the
   // Iterator interface in terms of a single ._invoke method.
@@ -370,10 +390,6 @@
   // unified ._invoke helper method.
   defineIteratorMethods(Gp);
 
-  Gp[iteratorSymbol] = function() {
-    return this;
-  };
-
   Gp[toStringTagSymbol] = "Generator";
 
   Gp.toString = function() {

test/non-native.js

@@ -12,9 +12,9 @@ describe("@@iterator", function() {
     var iterator = gen();
     assert.ok(!iterator.hasOwnProperty(Symbol.iterator));
     assert.ok(!Object.getPrototypeOf(iterator).hasOwnProperty(Symbol.iterator));
-    assert.ok(Object.getPrototypeOf(
+    assert.ok(Object.getPrototypeOf(Object.getPrototypeOf(
       Object.getPrototypeOf(iterator)
-    ).hasOwnProperty(Symbol.iterator));
+    )).hasOwnProperty(Symbol.iterator));
     assert.strictEqual(iterator[Symbol.iterator](), iterator);
   });
 });