--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -1404,16 +1404,33 @@ JS_IdentifyClassPrototype(JSContext *cx,
 {
     AssertHeapIsIdle(cx);
     CHECK_REQUEST(cx);
     assertSameCompartment(cx, obj);
     JS_ASSERT(!obj->is<CrossCompartmentWrapperObject>());
     return js_IdentifyClassPrototype(obj);
 }
 
+extern JS_PUBLIC_API(JSProtoKey)
+JS_IdToProtoKey(JSContext *cx, JS::HandleId id)
+{
+    AssertHeapIsIdle(cx);
+    CHECK_REQUEST(cx);
+
+    if (!JSID_IS_ATOM(id))
+        return JSProto_Null;
+    RootedString idstr(cx, JSID_TO_STRING(id));
+    const JSStdName *stdnm = LookupStdName(cx->runtime(), idstr, standard_class_names);
+    if (!stdnm)
+        return JSProto_Null;
+
+    MOZ_ASSERT(MOZ_ARRAY_LENGTH(standard_class_names) == JSProto_LIMIT + 1);
+    return static_cast<JSProtoKey>(stdnm - standard_class_names);
+}
+
 JS_PUBLIC_API(JSObject *)
 JS_GetObjectPrototype(JSContext *cx, JSObject *forObj)
 {
     CHECK_REQUEST(cx);
     assertSameCompartment(cx, forObj);
     return forObj->global().getOrCreateObjectPrototype(cx);
 }
 

--- a/js/src/jsapi.h
+++ b/js/src/jsapi.h
@@ -1764,16 +1764,19 @@ extern JS_PUBLIC_API(bool)
 JS_GetClassObject(JSContext *cx, JSObject *obj, JSProtoKey key, JSObject **objp);
 
 extern JS_PUBLIC_API(bool)
 JS_GetClassPrototype(JSContext *cx, JSProtoKey key, JSObject **objp);
 
 extern JS_PUBLIC_API(JSProtoKey)
 JS_IdentifyClassPrototype(JSContext *cx, JSObject *obj);
 
+extern JS_PUBLIC_API(JSProtoKey)
+JS_IdToProtoKey(JSContext *cx, JS::HandleId id);
+
 /*
  * Returns the original value of |Function.prototype| from the global object in
  * which |forObj| was created.
  */
 extern JS_PUBLIC_API(JSObject *)
 JS_GetFunctionPrototype(JSContext *cx, JSObject *forObj);
 
 /*