CLJS-869: When preamble is not found in source directory, compiler do…

…es not report it

src/clj/cljs/analyzer.clj

@@ -34,7 +34,8 @@
 (def -cljs-macros-loaded (atom false))
 
 (def ^:dynamic *cljs-warnings*
-  {:unprovided true
+  {:preamble-missing true
+   :unprovided true
    :undeclared-var false
    :undeclared-ns false
    :undeclared-ns-form true
@@ -60,9 +61,13 @@
 
 (defmulti error-message (fn [warning-type & _] warning-type))
 
+(defmethod error-message :preamble-missing
+  [warning-type info]
+  (str "Preamble resource file not found: " (string/join " " (:missing info))))
+
 (defmethod error-message :unprovided
   [warning-type info]
-  (str "Required namespace not provided for " (clojure.string/join " " (:unprovided info))))
+  (str "Required namespace not provided for " (string/join " " (:unprovided info))))
 
 (defmethod error-message :undeclared-var
   [warning-type info]

src/clj/cljs/closure.clj

@@ -523,7 +523,10 @@ should contain the source for the given namespace name."
                    inputs)))))
 
 (defn preamble-from-paths [paths]
-  (str (apply str (map #(slurp (io/resource %)) paths)) "\n"))
+  (when-let [missing (seq (remove io/resource paths))]
+    (ana/warning :preamble-missing @env/*compiler* {:missing (sort missing)}))
+  (let [resources (remove nil? (map io/resource paths))]
+    (str (string/join (map slurp resources)) "\n")))
 
 (defn make-preamble [{:keys [target preamble hashbang]}]
   (str (when (and (= :nodejs target) (not (false? hashbang)))