delete debug only files from output in release mode

pkg/polymer/lib/src/build/delete_file.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.
+
+/// Transformer that deletes everything that it sees, but only in release mode.
+library web_components.src.delete_file;
+
+import 'dart:async';
+import 'package:barback/barback.dart';
+
+// Deletes all files supplied in release mode.
+class DeleteFile extends Transformer {
+  BarbackSettings settings;
+
+  DeleteFile.asPlugin(this.settings);
+
+  /// Only apply to files in release mode.
+  isPrimary(_) => settings.mode == BarbackMode.RELEASE;
+
+  apply(Transform transform) {
+    transform.consumePrimary();
+  }
+}