Update ObjectToString to Harmony-draft algorithm

src/generator.js

Index: src/generator.js
diff --git a/src/generator.js b/src/generator.js
index 72e64dce6e852cf032b13c06a1ee091a2b6e8be0..40f151ca4017d4be092d4821e4be54a61978e0dd 100644
--- a/src/generator.js
+++ b/src/generator.js
@@ -68,12 +68,13 @@ function SetUpGenerators() {
                    DONT_ENUM | DONT_DELETE | READ_ONLY,
                    ["next", GeneratorObjectNext,
                     "throw", GeneratorObjectThrow]);
-
   %FunctionSetName(GeneratorObjectIterator, '[Symbol.iterator]');
   %AddNamedProperty(GeneratorObjectPrototype, symbolIterator,
       GeneratorObjectIterator, DONT_ENUM | DONT_DELETE | READ_ONLY);
   %AddNamedProperty(GeneratorObjectPrototype, "constructor",
       GeneratorFunctionPrototype, DONT_ENUM | DONT_DELETE | READ_ONLY);
+  %AddNamedProperty(GeneratorObjectPrototype,
+      symbolToStringTag, "Generator", DONT_ENUM | READ_ONLY);
   %InternalSetPrototype(GeneratorFunctionPrototype, $Function.prototype);
   %SetCode(GeneratorFunctionPrototype, GeneratorFunctionPrototypeConstructor);
   %AddNamedProperty(GeneratorFunctionPrototype, "constructor",