Bug 666399 - New Harmony syntax for generators. r=Waldo

Add a GeneratorKind enumeration, and use it in the parser and runtime to
indicate whether a function is a non-generator, a legacy generator, or a
star generator.

Always parse "yield" as TOK_YIELD, regardless of the version.  Add
TokenStream::currentName() to retrieve the current token's name, which
works for TOK_NAME or TOK_YIELD.  The parser should check the validity
of "yield" as a name, if needed, using checkYieldNameValidity().

Parse "function*" as the StarGenerator GeneratorKind, and allow star
generators to be lazily parsed.

Separate parsing of return statements from yield expressions.

js/src/jsiter.cpp

@@ -1475,6 +1475,13 @@ js_NewGenerator(JSContext *cx, const FrameRegs &stackRegs)
     JS_ASSERT(stackRegs.stackDepth() == 0);
     StackFrame *stackfp = stackRegs.fp();
 
+    JS_ASSERT(stackfp->script()->isGenerator());
+
+    if (stackfp->script()->isStarGenerator()) {
+        JS_ReportErrorNumber(cx, js_GetErrorMessage, NULL, JSMSG_ES6_UNIMPLEMENTED);
+        return NULL;
+    }
+
     Rooted<GlobalObject*> global(cx, &stackfp->global());
     RootedObject obj(cx);
     {