fixes #182, avoid injecting a duplicate link rel=stylesheet

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 'dart:collection' show HashSet, HashMap;
 
 import 'package:analyzer/analyzer.dart' show parseDirectives;
 import 'package:analyzer/src/generated/ast.dart'
     show
         AstNode,
         CompilationUnit,
         ExportDirective,
         Identifier,
         ImportDirective,
         LibraryDirective,
         PartDirective,
         PartOfDirective;
 import 'package:analyzer/src/generated/engine.dart'
     show ParseDartTask, AnalysisContext;
 import 'package:analyzer/src/generated/source.dart' show Source, SourceKind;
-import 'package:html/dom.dart' show Document, Node;
+import 'package:html/dom.dart' show Document, Node, Element;
 import 'package:html/parser.dart' as html;
 import 'package:logging/logging.dart' show Logger, Level;
 import 'package:path/path.dart' as path;
 
 import 'info.dart';
 import 'options.dart';
 import 'report.dart';
 
 /// Holds references to all source nodes in the import graph. This is mainly
 /// used as a level of indirection to ensure that each source has a canonical
@@ skipping 125 matching lines @@
   @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;
 
+  /// Tracks resource files referenced from HTML nodes, e.g.
+  /// `<link rel=stylesheet href=...>` and `<img src=...>`
+  final htmlResourceNodes = new HashMap<Element, ResourceSourceNode>();
+
   HtmlSourceNode(SourceGraph graph, Uri uri, Source source)
       : runtimeDeps = graph.runtimeDeps,
         super(graph, uri, source);
 
   @override
   void update() {
     super.update();
     if (needsRebuild) {
       graph._reporter.clearHtml(uri);
       document = html.parse(contents, generateSpans: true);
@@ skipping 12 matching lines @@
           _reportError(graph, 'Script file $src not found', script);
         }
         if (node != null) newScripts.add(node);
       }
 
       if (!_same(newScripts, scripts)) {
         structureChanged = true;
         scripts = newScripts;
       }
 
+      // TODO(jmesserly): simplify the design here. Ideally we wouldn't need
+      // to track user-defined CSS, images, etc. Also we don't have a clear
+      // way to distinguish runtime injected resources, like messages.css, from
+      // user-defined files.
+      htmlResourceNodes.clear();
       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'])));
+        var res = graph.nodeFromUri(uri.resolve(tag.attributes['href']));
+        htmlResourceNodes[tag] = res;
+        newResources.add(res);
       }
-      for (var tag in document.querySelectorAll('img')) {
-        newResources.add(graph.nodeFromUri(uri.resolve(tag.attributes['src'])));
+      for (var tag in document.querySelectorAll('img[src]')) {
+        var res = graph.nodeFromUri(uri.resolve(tag.attributes['src']));
+        htmlResourceNodes[tag] = res;
+        newResources.add(res);
       }
       if (!_same(newResources, resources)) {
         structureChanged = true;
         resources = newResources;
       }
     }
   }
 
   void _reportError(SourceGraph graph, String message, Node node) {
     graph._reporter.enterHtml(_source.uri);
@@ skipping 294 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');