Adding barback examples.

pkg/barback/examples/simple_transformer/lib/transformer.dart

Index: pkg/barback/examples/simple_transformer/lib/transformer.dart
diff --git a/pkg/barback/examples/simple_transformer/lib/transformer.dart b/pkg/barback/examples/simple_transformer/lib/transformer.dart
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+++ b/pkg/barback/examples/simple_transformer/lib/transformer.dart
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+// Copyright (c) 2012, 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 InsertCopyright extends Transformer {
+  String copyright = "Copyright (c) 2014, the Example project authors.\n";
+
+  // A constructor named "asPlugin" is required. It can be empty, but
+  // it must be present. It is how pub determines that you want this
+  // class to be publicly available as a loadable transformer plugin.
+  InsertCopyright.asPlugin();
+
+  Future<bool> isPrimary(Asset input) {
+    return new Future.value(input.id.extension == '.txt');
+  }
+
+  Future apply(Transform transform) {
+    return transform.primaryInput.readAsString().then((content) {
+      var id = transform.primaryInput.id;
+      String newContent = copyright + content;
+      transform.addOutput(new Asset.fromString(id, newContent));
+    });
+  }
+}