fs: Removing inStatWatchers function

We can get rid of `inStatWatchers` function by creating `statWatchers`
 with `null` as prototype and use `in` operator to check if `filename`
 is in it or not.

lib/fs.js

@@ -1297,12 +1297,7 @@ StatWatcher.prototype.stop = function() {
 };
 
 
-var statWatchers = {};
-function inStatWatchers(filename) {
-  return Object.prototype.hasOwnProperty.call(statWatchers, filename) &&
-      statWatchers[filename];
-}
-
+const statWatchers = Object.create(null);
 
 fs.watchFile = function(filename) {
   nullCheck(filename);
@@ -1329,7 +1324,7 @@ fs.watchFile = function(filename) {
     throw new Error('watchFile requires a listener function');
   }
 
-  if (inStatWatchers(filename)) {
+  if (filename in statWatchers) {
     stat = statWatchers[filename];
   } else {
     stat = statWatchers[filename] = new StatWatcher();
@@ -1342,7 +1337,7 @@ fs.watchFile = function(filename) {
 fs.unwatchFile = function(filename, listener) {
   nullCheck(filename);
   filename = pathModule.resolve(filename);
-  if (!inStatWatchers(filename)) return;
+  if (!(filename in statWatchers)) return;
 
   var stat = statWatchers[filename];
 
@@ -1354,7 +1349,7 @@ fs.unwatchFile = function(filename, listener) {
 
   if (EventEmitter.listenerCount(stat, 'change') === 0) {
     stat.stop();
-    statWatchers[filename] = undefined;
+    delete statWatchers[filename];
   }
 };