fs.watch should not require a listener arguments

Since fs.watch returns an event emitter where the change event is exactly
the same as the listener callback, the argument should be required

doc/api/fs.markdown

@@ -477,7 +477,7 @@ you need to compare `curr.mtime` and `prev.mtime`.
 
 Stop watching for changes on `filename`.
 
-## fs.watch(filename, [options], listener)
+## fs.watch(filename, [options], [listener])
 
     Stability: 2 - Unstable.  Not available on all platforms.
 

lib/fs.js

@@ -773,16 +773,15 @@ fs.watch = function(filename) {
     listener = arguments[1];
   }
 
-  if (!listener) {
-    throw new Error('watch requires a listener function');
-  }
-
   if (options.persistent === undefined) options.persistent = true;
 
   watcher = new FSWatcher();
   watcher.start(filename, options.persistent);
 
+  if (listener) {
     watcher.addListener('change', listener);
+  }
+
   return watcher;
 };
 

test/simple/test-fs-watch.js

@@ -58,18 +58,10 @@ try { fs.rmdirSync(testsubdir); } catch (e) { }
 
 fs.writeFileSync(filepathOne, 'hello');
 
-assert.throws(
-    function() {
-      fs.watch(filepathOne);
-    },
-    function(e) {
-      return e.message === 'watch requires a listener function';
-    }
-);
-
 assert.doesNotThrow(
     function() {
-      var watcher = fs.watch(filepathOne, function(event, filename) {
+      var watcher = fs.watch(filepathOne)
+      watcher.on('change', function(event, filename) {
         assert.equal('change', event);
         if (expectFilePath) {
           assert.equal('watch.txt', filename);
@@ -91,15 +83,6 @@ process.chdir(testDir);
 
 fs.writeFileSync(filepathTwoAbs, 'howdy');
 
-assert.throws(
-    function() {
-      fs.watch(filepathTwo);
-    },
-    function(e) {
-      return e.message === 'watch requires a listener function';
-    }
-);
-
 assert.doesNotThrow(
     function() {
       var watcher = fs.watch(filepathTwo, function(event, filename) {