Fix library name (dartanalyzer was complaining in the bots)

lib/src/span_with_context.dart

Index: lib/src/span_with_context.dart
diff --git a/lib/src/span_with_context.dart b/lib/src/span_with_context.dart
index 4d279de389f461f11e0ec5b5a80d34ce0514e9d1..edbb8b65ffe278f3084a4b8aa016c5b15df53a41 100644
--- a/lib/src/span_with_context.dart
+++ b/lib/src/span_with_context.dart
@@ -2,7 +2,7 @@
 // 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 source_span.span;
+library source_span.span_with_context;
 
 import 'location.dart';
 import 'span.dart';