Only pass an AssetId to isPrimary and declareOutputs.

samples/third_party/dromaeo/lib/transformer.dart

 library dromaeo.transformer;
 
 import 'dart:async';
 import 'package:barback/barback.dart';
 
 /// Transformer used by `pub build` and `pub serve` to rewrite dromaeo html
 /// files to run performance tests.
 class DromaeoTransformer extends Transformer {
 
   final BarbackSettings settings;
 
   DromaeoTransformer.asPlugin(this.settings);
 
   /// The index.html file and the tests/dom-*-html.html files are the ones we
   /// apply this transform to.
-  Future<bool> isPrimary(Asset asset) {
+  Future<bool> isPrimary(AssetId id) {
     var reg = new RegExp('(tests/dom-.+-html\.html\$)|(index\.html\$)');
-    return new Future.value(reg.hasMatch(asset.id.path));
+    return new Future.value(reg.hasMatch(id.path));
   }
 
   Future apply(Transform transform) {
     Asset primaryAsset = transform.primaryInput;
     AssetId primaryAssetId = primaryAsset.id;
 
     return primaryAsset.readAsString().then((String fileContents) {
       var filename = primaryAssetId.toString();
       var outputFileContents = fileContents;
 
@@ skipping 22 matching lines @@
   /// Given an html file that sources a Dart file, rewrite the html to instead
   /// source the compiled JavaScript file.
   String _sourceJsNotDart(String fileContents) {
     var dartScript = new RegExp(
         '<script type="application/dart" src="([\\w-]+)\.dart">');
     var match = dartScript.firstMatch(fileContents);
     return fileContents.replaceAll(dartScript, '<script type="text/javascript"'
         ' src="${match.group(1)+ ".dart.js"}" defer>');
   }
 }