Fix checked mode error

lib/src/dependency_graph.dart

 // 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.
 
 /// Tracks the shape of the import/export graph and dependencies between files.
 library dev_compiler.src.dependency_graph;
 
 import 'dart:collection' show HashSet;
 
 import 'package:analyzer/analyzer.dart' show parseDirectives;
@@ skipping 136 matching lines @@
 }
 
 /// A node representing an entry HTML source file.
 class HtmlSourceNode extends SourceNode {
   /// Resources included by default on any application.
   final runtimeDeps;
 
   /// Libraries referred to via script tags.
   Set<DartSourceNode> scripts = new Set<DartSourceNode>();
 
-  /// Link-rel stylesheets and images.
-  Set<ResourceSourceNode> resources = new Set<ResourceSourceNode>();
+  /// Link-rel stylesheets, images, and other specified files.
+  Set<SourceNode> resources = new Set<SourceNode>();
 
   @override
   Iterable<SourceNode> get allDeps =>
       [scripts, resources, runtimeDeps].expand((e) => e);
 
   @override
   Iterable<SourceNode> get depsWithoutParts => allDeps;
 
   /// Parsed document, updated whenever [update] is invoked.
   Document document;
@@ skipping 23 matching lines @@
           _reportError(graph, 'Script file $src not found', script);
         }
         if (node != null) newScripts.add(node);
       }
 
       if (!_same(newScripts, scripts)) {
         structureChanged = true;
         scripts = newScripts;
       }
 
-      var newResources = new Set<ResourceSourceNode>();
+      var newResources = new Set<SourceNode>();
       for (var resource in graph.resources) {
         newResources.add(graph.nodeFromUri(uri.resolve(resource)));
       }
       for (var tag in document.querySelectorAll('link[rel="stylesheet"]')) {
         newResources
             .add(graph.nodeFromUri(uri.resolve(tag.attributes['href'])));
       }
       for (var tag in document.querySelectorAll('img')) {
         newResources.add(graph.nodeFromUri(uri.resolve(tag.attributes['src'])));
       }
@@ skipping 303 matching lines @@
   'dart._interceptors',
   'dart._native_typed_data',
   */
 ];
 
 /// Runtime files added to applications when running in server mode.
 final runtimeFilesForServerMode = new List<String>.from(defaultRuntimeFiles)
   ..addAll(const ['messages_widget.js', 'messages.css']);
 
 final _log = new Logger('dev_compiler.dependency_graph');