Support source_span spans in barback.

sdk/lib/_internal/pub/test/build/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';
 
 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 27 matching lines @@
       d.dir(appPath, [
         d.dir('build', [
           d.dir('web', [
             d.file('foo.out', 'depeche')
           ])
         ])
       ]).validate();
     });
   });
 }