CLJS-1455: high resolution time

Added `cljs.core/system-time`, `time` macro now uses this instead

Author: dnolen

src/main/cljs/cljs/core.cljs

@@ -333,6 +333,15 @@
 
 (def DEMUNGE_PATTERN nil)
 
+(defn system-time
+  "Returns highest resolution time offered by host in milliseconds."
+  []
+  (cond
+    (exists? js/performance) (.now js/performance)
+    (exists? js/process) (let [t (.hrtime js/process)]
+                           (/ (+ (* (aget t 0) 1e9) (aget t 1)) 1e6))
+    :else (.getTime (js/Date.))))
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; arrays ;;;;;;;;;;;;;;;;
 
 (defn ^array make-array

src/main/clojure/cljs/core.cljc

@@ -2505,9 +2505,11 @@
 (core/defmacro time
   "Evaluates expr and prints the time it took. Returns the value of expr."
   [expr]
-  `(let [start# (.getTime (js/Date.))
+  `(let [start# (system-time)
          ret# ~expr]
-     (prn (cljs.core/str "Elapsed time: " (- (.getTime (js/Date.)) start#) " msecs"))
+     (prn (cljs.core/str "Elapsed time: "
+            (.toFixed (- (system-time) start#) 6)
+            " msecs"))
      ret#))
 
 (core/defmacro simple-benchmark