ObjMap, return original map if new kv is identical

Author: swannodette

src/main/cljs/cljs/core.cljs

@@ -12603,15 +12603,19 @@ reduces them without incurring seq initialization"
     (let [k (if-not (keyword? k) k (keyword->obj-map-key k))]
       (if (string? k)
         (if-not (nil? (scan-array 1 k strkeys))
-          (let [new-strobj (obj-clone strobj strkeys)]
-            (gobject/set new-strobj k v)
-            (ObjMap. meta strkeys new-strobj nil))             ;overwrite
-          (let [new-strobj (obj-clone strobj strkeys)          ; append
+          (if (identical? v (gobject/get strobj k))
+            coll
+            ;; overwrite
+            (let [new-strobj (obj-clone strobj strkeys)]
+              (gobject/set new-strobj k v)
+              (ObjMap. meta strkeys new-strobj nil))) 
+          ;; append
+          (let [new-strobj (obj-clone strobj strkeys) 
                 new-keys (aclone strkeys)]
             (gobject/set new-strobj k v)
             (.push new-keys k)
             (ObjMap. meta new-keys new-strobj nil)))
-        ; non-string key. game over.
+        ;; non-string key. game over.
         (-with-meta
           (-kv-reduce coll
             (fn [ret k v]