CLJS-461: preserve metadata on automatic map conversions

Also includes basic tests based on the failing form from the ticket.

src/cljs/cljs/core.cljs

@@ -3597,13 +3597,13 @@ reduces them without incurring seq initialization"
   (let [ks  (.-keys m)
         len (alength ks)
         so  (.-strobj m)
-        out (with-meta cljs.core.PersistentHashMap/EMPTY (meta m))]
+        mm  (meta m)]
     (loop [i   0
-           out (transient out)]
+           out (transient cljs.core.PersistentHashMap/EMPTY)]
       (if (< i len)
         (let [k (aget ks i)]
           (recur (inc i) (assoc! out k (aget so k))))
-        (persistent! (assoc! out k v))))))
+        (with-meta (persistent! (assoc! out k v)) mm)))))
 
 ;;; ObjMap
 
@@ -3912,10 +3912,9 @@ reduces them without incurring seq initialization"
                                  (.push k)
                                  (.push v))
                                nil)
-          (persistent!
-           (assoc!
-            (transient (into cljs.core.PersistentHashMap/EMPTY coll))
-            k v)))
+          (with-meta
+            (assoc (into cljs.core.PersistentHashMap/EMPTY coll) k v)
+            meta))
 
         (identical? v (aget arr (inc idx)))
         coll

test/cljs/cljs/core_test.cljs

@@ -1262,6 +1262,20 @@
           (assert (= (count m1) 100))
           (assert (= (count m2) 100)))))
 
+  ;; CLJS-461: automatic map conversions
+  (loop [i 0 m (with-meta {} {:foo :bar}) result []]
+    (if (<= i (+ cljs.core.ObjMap/HASHMAP_THRESHOLD 2))
+      (recur (inc i) (assoc m (str i) i) (conj result (meta m)))
+      (let [n (inc (+ cljs.core.ObjMap/HASHMAP_THRESHOLD 2))
+            expected (repeat n {:foo :bar})]
+        (assert (= result expected)))))
+  (loop [i 0 m (with-meta {-1 :quux} {:foo :bar}) result []]
+    (if (<= i (+ cljs.core.PersistentArrayMap/HASHMAP_THRESHOLD 2))
+      (recur (inc i) (assoc m i i) (conj result (meta m)))
+      (let [n (inc (+ cljs.core.PersistentArrayMap/HASHMAP_THRESHOLD 2))
+            expected (repeat n {:foo :bar})]
+        (assert (= result expected)))))
+
   ;; TransientHashSet
   (loop [s (transient #{})
          i 0]