a more generic example

start an example

BUG=
R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1350833002 .

example/hash.dart

@@ -0,0 +1,48 @@
+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:io' show exit, File;
+import 'package:crypto/crypto.dart' show MD5, SHA1, SHA256, CryptoUtils;
+
+const USAGE = 'Usage: dart hash.dart <md5|sha1|sha256> <input_filename>';
+
+main(List<String> args) async {
+  if (args == null || args.length != 2) {
+    print(USAGE);
+    exit(1);
+  }
+
+  var hasher;
+
+  switch (args[0]) {
+    case 'md5':
+      hasher = new MD5();
+      break;
+    case 'sha1':
+      hasher = new SHA1();
+      break;
+    case 'sha256':
+      hasher = new SHA256();
+      break;
+    default:
+      print(USAGE);
+      exit(1);
+  }
+
+  var filename = args[1];
+  var input = new File(filename);
+
+  if (!input.existsSync()) {
+    print('File "$filename" does not exist.');
+    exit(1);
+  }
+
+  await for (var bytes in input.openRead()) {
+    hasher.add(bytes);
+  }
+
+  var hex = CryptoUtils.bytesToHex(hasher.close());
+
+  print(hex);
+}