Generator objects have [[Class]] === "Generator"

src/generator.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 21 matching lines @@
 // var $Function = global.Function;
 
 // ----------------------------------------------------------------------------
 
 
 // TODO(wingo): Give link to specification. For now, the following diagram is
 // the spec:
 // http://wiki.ecmascript.org/lib/exe/fetch.php?cache=cache&media=harmony:es6_generator_object_model_3-29-13.png
 
 function GeneratorObjectNext() {
+  if (!IS_GENERATOR(this)) {
+    throw MakeTypeError('incompatible_method_receiver',
+                        ['[Generator].prototype.next', this]);
+  }
+
   // TODO(wingo): Implement.
 }
 
 function GeneratorObjectSend(value) {
+  if (!IS_GENERATOR(this)) {
+    throw MakeTypeError('incompatible_method_receiver',
+                        ['[Generator].prototype.send', this]);
+  }
+
   // TODO(wingo): Implement.
 }
 
 function GeneratorObjectThrow(exn) {
+  if (!IS_GENERATOR(this)) {
+    throw MakeTypeError('incompatible_method_receiver',
+                        ['[Generator].prototype.throw', this]);
+  }
+
   // TODO(wingo): Implement.
 }
 
 function GeneratorObjectClose() {
+  if (!IS_GENERATOR(this)) {
+    throw MakeTypeError('incompatible_method_receiver',
+                        ['[Generator].prototype.close', this]);
+  }
+
   // TODO(wingo): Implement.
 }
 
 function SetUpGenerators() {
   %CheckIsBootstrapping();
   var GeneratorObjectPrototype = GeneratorFunctionPrototype.prototype;
   InstallFunctions(GeneratorObjectPrototype,
                    DONT_ENUM | DONT_DELETE | READ_ONLY,
                    ["next", GeneratorObjectNext,
                     "send", GeneratorObjectSend,
                     "throw", GeneratorObjectThrow,
                     "close", GeneratorObjectClose]);
   %SetProperty(GeneratorObjectPrototype, "constructor",
                GeneratorFunctionPrototype, DONT_ENUM | DONT_DELETE | READ_ONLY);
   %SetPrototype(GeneratorFunctionPrototype, $Function.prototype);
   %SetProperty(GeneratorFunctionPrototype, "constructor",
                GeneratorFunction, DONT_ENUM | DONT_DELETE | READ_ONLY);
   %SetPrototype(GeneratorFunction, $Function);
 }
 
 SetUpGenerators();