Support source_span spans in barback.

sdk/lib/_internal/pub/test/serve/allows_arbitrary_modes_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.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 pub_tests;
 
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import '../serve/utils.dart';
 
 const TRANSFORMER = """
 import 'dart:async';
 
 import 'package:barback/barback.dart';
-import 'package:source_maps/source_maps.dart';
 
 class ModeTransformer extends Transformer {
   final BarbackSettings settings;
   ModeTransformer.asPlugin(this.settings);
 
   String get allowedExtensions => '.txt';
 
   Future apply(Transform transform) {
     return new Future.value().then((_) {
       var id = transform.primaryInput.id.changeExtension(".out");
@@ skipping 21 matching lines @@
       ]).create();
 
       createLockFile('myapp', pkg: ['barback']);
 
       pubServe(args: ["--mode", "depeche"]);
       requestShouldSucceed("foo.out", "depeche");
       endPubServe();
     });
   });
 }