Adding a lazy transformer example.

pkg/barback/example/lazy_transformer/lib/transformer.dart

+// Copyright (c) 2014, 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 'package:barback/barback.dart';
+
+import 'dart:async';
+
+class CodedMessageConverter extends Transformer 
+                            implements LazyTransformer {
+
+  // A constructor named "asPlugin" is required. It can be empty, but
+  // it must be present.
+  CodedMessageConverter.asPlugin();
+
+  Future<bool> isPrimary(AssetId id) {
+    return new Future.value(id.extension == '.txt');
+  }
+
+  Future declareOutputs(DeclaringTransform transform) {
+    return new Future.value(transform.primaryId.changeExtension('.shhhhh'));
+  }
+
+  Future apply(Transform transform) {
+    return transform.primaryInput.readAsString().then((content) {
+
+      // The output file is created with the '.shhhhh' extension.
+      var id = transform.primaryInput.id.changeExtension('.shhhhh');
+
+      StringBuffer newContent = new StringBuffer();
+      for (int i = 0; i < content.length; i++ ) {
+        newContent.write(rot13(content[i]));
+      }
+      transform.addOutput(new Asset.fromString(id, newContent.toString()));
+    });
+  }
+
+  rot13(var ch) {
+    var c = ch.codeUnitAt(0);
+    if      (c >= 'a'.codeUnitAt(0) && c <= 'm'.codeUnitAt(0)) c += 13;
+    else if (c >= 'A'.codeUnitAt(0) && c <= 'M'.codeUnitAt(0)) c += 13;
+    else if (c >= 'n'.codeUnitAt(0) && c <= 'z'.codeUnitAt(0)) c -= 13;
+    else if (c >= 'N'.codeUnitAt(0) && c <= 'Z'.codeUnitAt(0)) c -= 13;
+    return new String.fromCharCode(c);
+  }
+}