Fix an import in yaml_node_wrapper_test.

pkg/yaml/test/yaml_node_wrapper_test.dart

 // Copyright (c) 2012, 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 yaml.node_wrapper_test;
 
-import 'package:source_maps/source_maps.dart';
+import 'package:source_span/source_span.dart';
 import 'package:unittest/unittest.dart';
 import 'package:yaml/yaml.dart';
 
 main() {
   test("YamlMap() with no sourceUrl", () {
     var map = new YamlMap();
     expect(map, isEmpty);
     expect(map.nodes, isEmpty);
     expect(map.span, isNullSpan(isNull));
   });
@@ skipping 136 matching lines @@
   expect(span, new isInstanceOf<SourceSpan>());
   expect(span.length, equals(0));
   expect(span.text, isEmpty);
   expect(span.start, equals(span.end));
   expect(span.start.offset, equals(0));
   expect(span.start.line, equals(0));
   expect(span.start.column, equals(0));
   expect(span.sourceUrl, sourceUrl);
   return true;
 });