Extract out a source_span package from source_maps.

pkg/source_span/test/utils_test.dart

Index: pkg/source_span/test/utils_test.dart
diff --git a/pkg/source_maps/test/utils_test.dart b/pkg/source_span/test/utils_test.dart
similarity index 91%
copy from pkg/source_maps/test/utils_test.dart
copy to pkg/source_span/test/utils_test.dart
index 79a7de76903826410c1167d14f1733e3b742e38b..39211111e7c4b358dc253411c578618f586b8f12 100644
--- a/pkg/source_maps/test/utils_test.dart
+++ b/pkg/source_span/test/utils_test.dart
@@ -2,11 +2,8 @@
 // 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.
 
-/// Tests for the binary search utility algorithm.
-library test.utils_test;
-
 import 'package:unittest/unittest.dart';
-import 'package:source_maps/src/utils.dart';
+import 'package:source_span/src/utils.dart';
 
 main() {
   group('binary search', () {