CLJS-2862: Externs inference warning when extending Object

extend-type macro modifies the prototype with one helper.
Just add meta to the returned form and check that case
in 'set! analysis.

Author: swannodette

src/main/clojure/cljs/analyzer.cljc

@@ -2514,7 +2514,13 @@
 
                       :else
                       (when (seq? target)
-                        (let [texpr (analyze-seq enve target nil)]
+                        (let [texpr (if (-> target meta :extend-type)
+                                      ;; we're setting a prototype via extend-type macro
+                                      ;; nothing to warn
+                                      (binding [*cljs-warnings*
+                                                (assoc *cljs-warnings* :infer-warning false)]
+                                        (analyze-seq enve target nil))
+                                      (analyze-seq enve target nil))]
                           (when (:field texpr)
                             texpr))))
               vexpr (analyze enve val)]

src/main/clojure/cljs/core.cljc

@@ -1461,7 +1461,8 @@
   [tsym sym] `(.. ~tsym ~(to-property sym)))
 
 (core/defmethod extend-prefix :default
-  [tsym sym] `(.. ~tsym ~'-prototype ~(to-property sym)))
+  [tsym sym]
+  (with-meta `(.. ~tsym ~'-prototype ~(to-property sym)) {:extend-type true}))
 
 (core/defn- adapt-obj-params [type [[this & args :as sig] & body]]
   (core/list (vec args)

src/test/clojure/cljs/externs_infer_tests.clj

@@ -9,7 +9,7 @@
     [cljs.test-util :refer [unsplit-lines]]
     [cljs.util :as util]
     [clojure.string :as string]
-    [clojure.test :refer [is are deftest testing]]))
+    [clojure.test :as test :refer [is are deftest testing]]))
 
 (def externs-cenv
   (atom