re-implement figwheel.client.utils/feature? as a macro

The previous implementation wrapped cljs.core/exists? (a macro) in a function, meaning that a ReferenceError would throw before exists? can do its thing.

support/src/figwheel/client/utils.clj

@@ -18,3 +18,8 @@
 (defmacro dev-assert [& body]
  `(dev
  ~@(map (fn [pred-stmt] `(assert ~pred-stmt)) body)))
+
+(defmacro feature?
+ [obj feature]
+ `(and (cljs.core/exists? ~obj)
+ (cljs.core/exists? (cljs.core/aget ~obj ~feature))))

support/src/figwheel/client/utils.cljs

@@ -7,7 +7,8 @@
  [goog.userAgent.product :as product])
  (:import [goog]
  [goog.async Deferred]
- [goog.string StringBuffer]))
+ [goog.string StringBuffer])
+ (:require-macros [figwheel.client.utils :refer [feature?]]))
 
 ;; don't auto reload this file it will mess up the debug printing
 
@@ -110,19 +111,11 @@
  deferred coll)
  (fn [_] (.succeed Deferred @results)))))
 
-
-(defn- feature? [obj feature]
- (and (exists? obj)
- (exists? (aget obj feature))))
-
-
 ;; persistent storage of configuration keys
 
 (defonce local-persistent-config
  (let [a (atom {})]
- (when (case (host-env?)
- :worker (feature? (.-localStorage js/self) "setItem")
- (feature? js/localStorage "setItem"))
+ (when (feature? js/localStorage "setItem")
  (add-watch a :sync-local-storage
  (fn [_ _ _ n]
  (mapv (fn [[ky v]]