Switch from interfaces to abstract classes in dart:io.

pkg/dartdoc/lib/dartdoc.dart

 // 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.
 
 /**
  * To generate docs for a library, run this script with the path to an
  * entrypoint .dart file, like:
  *
  *     $ dart dartdoc.dart foo.dart
  *
@@ skipping 137 matching lines @@
 
   /**
    * Generates the App Cache manifest file, enabling offline doc viewing.
    */
   bool generateAppCache = false;
 
   /** Path to the dartdoc directory. */
   Path dartdocPath;
 
   /** Path to generate HTML files into. */
-  Path outputDir = const Path('docs');
+  Path outputDir = new Path('docs');
 
   /**
    * The title used for the overall generated output. Set this to change it.
    */
   String mainTitle = 'Dart Documentation';
 
   /**
    * The URL that the Dart logo links to. Defaults "index.html", the main
    * page for the generated docs, but can be anything.
    */
@@ skipping 1398 matching lines @@
 
 /**
  * Used to report an unexpected error in the DartDoc tool or the
  * underlying data
  */
 class InternalError {
   final String message;
   const InternalError(this.message);
   String toString() => "InternalError: '$message'";
 }