stop using dart2dart in Makefile

prepend.dart

-#!/usr/bin/env dart
-import 'dart:io';
-
-main(List<String> arguments) {
-  final fileName = arguments.first;
-  final file = new File(fileName);
-  final content = file.readAsStringSync();
-  file.writeAsStringSync('#!/usr/bin/env dart\n$content');
-  // For development use the line below instead of the one above to
-  // avoid using the dart2dart code.
-  // file.writeAsStringSync('#!/bin/bash\ndart bin/protoc_plugin.dart');
-}