CLJS-1168: REPL fails to find .js files in :libs

Improve undeclared ns error reporting to account for :libs. :libs IJavaScript
instance do not supply :file only :url. Enhance cljs.closure/source-for-namespace
so it handles IJavaScript instances that do not supply :file.

src/main/clojure/cljs/analyzer.cljc

@@ -109,10 +109,11 @@
   (str "Use of undeclared Var " (:prefix info) "/" (:suffix info)))
 
 (defmethod error-message :undeclared-ns
-  [warning-type {:keys [ns-sym] :as info}]
+  [warning-type {:keys [ns-sym js-provide] :as info}]
   (str "No such namespace: " ns-sym
        ", could not locate " (util/ns->relpath ns-sym :cljs)
-       " or " (util/ns->relpath ns-sym :cljc)))
+       ", " (util/ns->relpath ns-sym :cljc)
+       ", or Closure namespace \"" js-provide "\""))
 
 (defmethod error-message :dynamic
   [warning-type info]
@@ -1290,7 +1291,7 @@
              (analyze-file src opts)
              (throw
                (error env
-                 (error-message :undeclared-ns {:ns-sym dep}))))))))))
+                 (error-message :undeclared-ns {:ns-sym dep :js-provide (name dep)}))))))))))
 
 (defn check-uses [uses env]
   (doseq [[sym lib] uses]

src/main/clojure/cljs/closure.clj

@@ -518,11 +518,13 @@
       (let [relpath (str path ".cljc")]
         (if-let [cljc-res (io/resource relpath)]
           {:relative-path relpath :uri cljc-res}
-          (let [relpath (:file (get-in @compiler-env [:js-dependency-index (str ns)]))]
+          (let [ijs (get-in @compiler-env [:js-dependency-index (str ns)])
+                relpath (or (:file ijs) (:url ijs))]
             (if-let [js-res (and relpath
                                  ;; try to parse URL, otherwise just return local
                                  ;; resource
-                                 (or (try (URL. relpath) (catch Throwable t))
+                                 (or (and (util/url? relpath) relpath)
+                                     (try (URL. relpath) (catch Throwable t))
                                      (io/resource relpath)))]
               {:relative-path relpath :uri js-res}
               (throw