add single files, secure server, hello world

single_file_tests/file_append.dart

@@ -0,0 +1,15 @@
+#!/usr/bin/env dart
+
+import 'dart:io';
+
+// https://juejin.im/post/5d7f4c35f265da03c50341df
+void main() async{
+  File file = new File('test.txt');
+  IOSink isk = file.openWrite(mode: FileMode.append);
+
+  isk.write('Line 1');
+  isk.writeln(', also line 1');
+  isk.writeln('Line 2');
+  await isk.close();
+  print('Done!');
+}