fs: remove inStatWatchers and use Map for lookup

Remove `inStatWatchers` function and make `statWatchers` a `Map`.

PR-URL: #1870
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

lib/fs.js

@@ -1301,12 +1301,7 @@ StatWatcher.prototype.stop = function() {
 };
 
 
-var statWatchers = {};
-function inStatWatchers(filename) {
-  return Object.prototype.hasOwnProperty.call(statWatchers, filename) &&
-      statWatchers[filename];
-}
-
+const statWatchers = new Map();
 
 fs.watchFile = function(filename) {
   nullCheck(filename);
@@ -1333,22 +1328,24 @@ fs.watchFile = function(filename) {
     throw new Error('watchFile requires a listener function');
   }
 
-  if (inStatWatchers(filename)) {
-    stat = statWatchers[filename];
-  } else {
-    stat = statWatchers[filename] = new StatWatcher();
+  stat = statWatchers.get(filename);
+
+  if (stat === undefined) {
+    stat = new StatWatcher();
     stat.start(filename, options.persistent, options.interval);
+    statWatchers.set(filename, stat);
   }
+
   stat.addListener('change', listener);
   return stat;
 };
 
 fs.unwatchFile = function(filename, listener) {
   nullCheck(filename);
   filename = pathModule.resolve(filename);
-  if (!inStatWatchers(filename)) return;
+  var stat = statWatchers.get(filename);
 
-  var stat = statWatchers[filename];
+  if (stat === undefined) return;
 
   if (typeof listener === 'function') {
     stat.removeListener('change', listener);
@@ -1358,7 +1355,7 @@ fs.unwatchFile = function(filename, listener) {
 
   if (EventEmitter.listenerCount(stat, 'change') === 0) {
     stat.stop();
-    statWatchers[filename] = undefined;
+    statWatchers.delete(filename);
   }
 };