Only run [declareOutputs] once for each asset/transformer pair.

pkg/barback/test/utils.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.
 
 library barback.test.utils;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:convert' show Encoding;
 
 import 'package:barback/barback.dart';
 import 'package:barback/src/cancelable_future.dart';
 import 'package:barback/src/utils.dart';
 import 'package:path/path.dart' as pathos;
 import 'package:scheduled_test/scheduled_test.dart';
 import 'package:stack_trace/stack_trace.dart';
 import 'package:unittest/compact_vm_config.dart';
 
 export 'transformer/bad.dart';
 export 'transformer/bad_log.dart';
 export 'transformer/catch_asset_not_found.dart';
 export 'transformer/check_content.dart';
 export 'transformer/check_content_and_rename.dart';
 export 'transformer/conditionally_consume_primary.dart';
 export 'transformer/create_asset.dart';
+export 'transformer/declare_assets.dart';
+export 'transformer/declaring_bad.dart';
+export 'transformer/declaring_rewrite.dart';
 export 'transformer/emit_nothing.dart';
 export 'transformer/has_input.dart';
 export 'transformer/lazy_bad.dart';
 export 'transformer/lazy_many_to_one.dart';
 export 'transformer/lazy_rewrite.dart';
 export 'transformer/many_to_one.dart';
 export 'transformer/mock.dart';
 export 'transformer/one_to_many.dart';
 export 'transformer/rewrite.dart';
 
@@ skipping 536 matching lines @@
 
   _MockAsset(this.id, this.contents);
 
   Future<String> readAsString({Encoding encoding}) =>
       new Future.value(contents);
 
   Stream<List<int>> read() => throw new UnimplementedError();
 
   String toString() => "MockAsset $id $contents";
 }