Fix checked mode error

R=jmesserly@google.com

Review URL: https://codereview.chromium.org/1167543002

Author: vsmenon

pkg/dev_compiler/lib/src/dependency_graph.dart

@@ -154,8 +154,8 @@ class HtmlSourceNode extends SourceNode {
   /// 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 =>
@@ -199,7 +199,7 @@ class HtmlSourceNode extends SourceNode {
         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)));
       }

pkg/dev_compiler/lib/src/options.dart

@@ -311,7 +311,10 @@ CompilerOptions parseOptions(List<String> argv) {
       useMultiPackage: args['use-multi-package'],
       packageRoot: args['package-root'],
       packagePaths: args['package-paths'].split(','),
-      resources: args['resources'].split(','),
+      resources: args['resources']
+          .split(',')
+          .where((s) => s.isNotEmpty)
+          .toList(),
       inferDownwards: args['infer-downwards'],
       inferFromOverrides: args['infer-from-overrides'],
       inferTransitively: args['infer-transitively'],