CLJS-2104: Const-replaced exprs do not emit js "return"

When const-replacement is done during emission, we must ensure we use
the :context of the *replaced* form, not the const form.

Author: favila

src/main/clojure/cljs/compiler.cljc

@@ -332,7 +332,7 @@
 (defmethod emit* :var
   [{:keys [info env form] :as ast}]
   (if-let [const-expr (:const-expr ast)]
-    (emit const-expr)
+    (emit (assoc const-expr :env env))
     (let [var-name (:name info)
           info (if (= (namespace var-name) "js")
                  (let [js-module-name (get-in @env/*compiler* [:js-module-index (name var-name)])]

src/test/cljs/cljs/core_test.cljs

@@ -1327,6 +1327,16 @@
     (is (= :fail (try (subvec nil 1 nil)
                       (catch js/Error e :fail))))))
 
+(def ^:const cljs-2104 "cljs-2104")
+
+(deftest test-const-emission
+  (testing "const exprs emission context, not definition context (CLJS-2104)"
+    (is (= cljs-2104 "cljs-2104"))
+    (is (= (if-some [x true]
+             cljs-2104
+             "unreachable")
+          "cljs-2104"))))
+
 (comment
   ;; ObjMap
   ;; (let [ks (map (partial str "foo") (range 500))