webassembly: Use Date's now() instead of getTime().

Using now() is a bit faster because it doesn't need to create a new Date.

Signed-off-by: Damien George <damien@micropython.org>

ports/webassembly/library.js

@@ -42,7 +42,7 @@ mergeInto(LibraryManager.library, {
     },
 
     mp_js_ticks_ms: function() {
-        return (new Date()).getTime() - MP_JS_EPOCH;
+        return Date.now() - MP_JS_EPOCH;
     },
 
     mp_js_hook: function() {

ports/webassembly/wrapper.js

@@ -33,7 +33,7 @@ var mainProgram = function()
   mp_js_init_repl = Module.cwrap('mp_js_init_repl', 'null', ['null']);
   mp_js_process_char = Module.cwrap('mp_js_process_char', 'number', ['number']);
 
-  MP_JS_EPOCH = (new Date()).getTime();
+  MP_JS_EPOCH = Date.now();
 
   if (typeof window === 'undefined' && require.main === module) {
       var fs = require('fs');