CLJS-3164: Optimize assoc on IAssociative values

Author: mfikes

src/main/cljs/cljs/core.cljs

@@ -1973,9 +1973,11 @@ reduces them without incurring seq initialization"
    val(s). When applied to a vector, returns a new vector that
    contains val at index. Note - index must be <= (count vector)."
   ([coll k v]
-    (if-not (nil? coll)
-      (-assoc coll k v)
-      (array-map k v)))
+   (if (implements? IAssociative coll)
+     (-assoc coll k v)
+     (if-not (nil? coll)
+       (-assoc coll k v)
+       (array-map k v))))
   ([coll k v & kvs]
      (let [ret (assoc coll k v)]
        (if kvs