Fix protocol fn DCE issue (#252)

* fixes invoke to cljs.core/str in protocol fns
* add another trivial dce test w/ a protocol fn

Author: swannodette

src/main/clojure/cljs/core.cljc

@@ -3287,9 +3287,9 @@
                            argseq#))))
                   (if (:macro meta)
                     `(throw (js/Error.
-                             (str "Invalid arity: " (- (alength (js-arguments)) 2))))
+                             (.join (array "Invalid arity: " (- (alength (js-arguments)) 2)) "")))
                     `(throw (js/Error.
-                             (str "Invalid arity: " (alength (js-arguments))))))))))
+                             (.join (array "Invalid arity: " (alength (js-arguments))) ""))))))))
          ~@(map #(fn-method name %) fdecl)
          ;; optimization properties
          (set! (. ~name ~'-cljs$lang$maxFixedArity) ~maxfa)

src/test/cljs_build/trivial/core2.cljs

@@ -0,0 +1,3 @@
+(ns trivial.core2)
+
+(. js/console (-lookup 1 2))

src/test/clojure/cljs/build_api_tests.clj

@@ -720,6 +720,19 @@
     (build/build (build/inputs (io/file inputs "trivial/core.cljs")) opts cenv)
     (is (< (.length out-file) 10000))))
 
+(deftest trivial-output-size-protocol
+  (let [out (.getPath (io/file (test/tmp-dir) "trivial-output-protocol-test-out"))
+        out-file (io/file out "main.js")
+        {:keys [inputs opts]} {:inputs (str (io/file "src" "test" "cljs_build"))
+                               :opts {:main 'trivial.core2
+                                      :output-dir out
+                                      :output-to (.getPath out-file)
+                                      :optimizations :advanced}}
+        cenv (env/default-compiler-env)]
+    (test/delete-out-files out)
+    (build/build (build/inputs (io/file inputs "trivial/core2.cljs")) opts cenv)
+    (is (< (.length out-file) 10000))))
+
 (deftest cljs-3255-nil-inputs-build
   (let [out (.getPath (io/file (test/tmp-dir) "3255-test-out"))
         out-file (io/file out "main.js")