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

src/generator.js

Index: src/generator.js
diff --git a/src/generator.js b/src/generator.js
index d579928cbae74531af80108f12f62ecd1fb4048b..481d4d37f88f4d2ce3541b5ae04ff13495a4c998 100644
--- a/src/generator.js
+++ b/src/generator.js
@@ -39,18 +39,38 @@
 // 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.
 }