Extract out a source_span package from source_maps.

pkg/source_span/lib/source_span.dart

Index: pkg/source_span/lib/source_span.dart
diff --git a/pkg/analysis_testing/test/test_all.dart b/pkg/source_span/lib/source_span.dart
similarity index 52%
copy from pkg/analysis_testing/test/test_all.dart
copy to pkg/source_span/lib/source_span.dart
index a6c2ed265842a6f7e33dbe33aa567db0a4915411..cb0302ab98c874df2ac8ab737ab42c92f510bfce 100644
--- a/pkg/analysis_testing/test/test_all.dart
+++ b/pkg/source_span/lib/source_span.dart
@@ -2,10 +2,10 @@
 // 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.
 
-//import 'package:unittest/unittest.dart';
+library source_span;
 
-/// Utility for manually running all tests.
-main() {
-//  group('analysis_services', () {
-//  });
-}
+export "src/source_file.dart";
+export "src/source_location.dart";
+export "src/source_span.dart";
+export "src/source_span_exception.dart";
+export "src/source_span_mixin.dart";