CLJS-477: Vararg fns break when first vararg is undefined.

Examine arguments.length to determine the presence of varargs instead of
checking the argument in the first vararg position with goog.isDef(arg).

src/clj/cljs/compiler.clj

@@ -422,7 +422,7 @@
                  (emitln "var self__ = this;"))
                (when variadic
                  (emitln "var " (last params) " = null;")
-                 (emitln "if (goog.isDef(var_args)) {")
+                 (emitln "if (arguments.length > " (dec (count params)) ") {")
                  (emitln "  " (last params) " = cljs.core.array_seq(Array.prototype.slice.call(arguments, " (dec (count params)) "),0);")
                  (emitln "} "))
                (emitln "return " delegate-name ".call(" (string/join ", " (cons "this" params)) ");")

test/cljs/cljs/core_test.cljs

@@ -1828,5 +1828,10 @@
   (assert (= (reduce-kv + 0 (apply hash-map (range 1000)))
              (reduce + (range 1000))))
 
+  ;; CLJS-477
+
+  (assert (= [js/undefined 1 2] ((fn [& more] more) js/undefined 1 2)))
+  (assert (= [js/undefined 4 5] ((fn [a b & more] more) 1 2 js/undefined 4 5)))
+
   :ok
   )