fs: Making SyncWriteStream non-enumerable

As `SyncWriteStream` is only for internal use, it would be better
if it is non-enumerable, so that a simple `console.log(require('fs'))`
will not reveal it.

lib/fs.js

@@ -1878,8 +1878,11 @@ util.inherits(SyncWriteStream, Stream);
 
 
 // Export
-fs.SyncWriteStream = SyncWriteStream;
-
+Object.defineProperty(fs, 'SyncWriteStream', {
+    configurable: true,
+    writable: true,
+    value: SyncWriteStream
+});
 
 SyncWriteStream.prototype.write = function(data, arg1, arg2) {
   var encoding, cb;