Make Transform a little more pleasant to use:

pkg/observe/test/transform_test.dart

 // Copyright (c) 2013, 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.
 
 import 'dart:async';
 import 'package:barback/barback.dart';
 import 'package:observe/transform.dart';
 import 'package:unittest/compact_vm_config.dart';
 import 'package:unittest/unittest.dart';
 
@@ skipping 87 matching lines @@
       expect(transform.outs, hasLength(1));
       expect(transform.outs[0].id, id);
       return transform.outs.first.readAsString();
     });
   });
 }
 
 class _MockTransform implements Transform {
   List<Asset> outs = [];
   Asset _asset;
-  AssetId get primaryId => _asset.id;
   TransformLogger logger = new TransformLogger(false);
-  Future<Asset> get primaryInput => new Future.value(_asset);
+  Asset get primaryInput => _asset;
 
   _MockTransform(this._asset);
   Future<Asset> getInput(Asset id) {
-    if (id == primaryId) return primaryInput;
+    if (id == primaryInput.id) return new Future.value(primaryInput);
     fail();
   }
 
   void addOutput(Asset output) {
     outs.add(output);
   }
 }