Don't require Transformer methods to return Futures.

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';
 
@@ skipping 10 matching lines @@
 export 'transformer/emit_nothing.dart';
 export 'transformer/has_input.dart';
 export 'transformer/lazy_assets.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';
+export 'transformer/sync_rewrite.dart';
 
 var _configured = false;
 
 MockProvider _provider;
 Barback _barback;
 
 /// Calls to [buildShouldSucceed] and [buildShouldFail] set expectations on
 /// successive [BuildResult]s from [_barback]. This keeps track of how many
 /// calls have already been made so later calls know which result to look for.
 int _nextBuildResult;
@@ skipping 531 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";
 }