CLJS-3395: (set! a -x false) doesn't work (#205)

Count the forms, add runtime test case

Author: swannodette

src/main/clojure/cljs/analyzer.cljc

@@ -2604,7 +2604,7 @@
 
 (defmethod parse 'set!
   [_ env [_ target val alt :as form] _ _]
-  (let [[target val] (if alt
+  (let [[target val] (if (= 4 (count form))
                        ;; (set! o -prop val)
                        [`(. ~target ~val) alt]
                        [target val])]

src/test/cljs/cljs/core_test.cljs

@@ -1987,3 +1987,9 @@
   (testing "macroexpanding non-seqs should work"
     (is (true? (macroexpand '(and))))
     (is (nil? (macroexpand '(or))))))
+
+(deftest test-cljs-3395
+  (testing "(set! foo -bar baz) pattern"
+    (let [a #js {}]
+      (set! a -x false)
+      (is (false? (.-x a))))))