Fix library name (dartanalyzer was complaining in the bots)

R=nweiz@google.com

Review URL: https://codereview.chromium.org//1033083002

pkgs/source_span/lib/src/span_with_context.dart

diff --git a/pkgs/source_span/lib/src/span_with_context.dart b/pkgs/source_span/lib/src/span_with_context.dart
index 4d279de..edbb8b6 100644
--- a/pkgs/source_span/lib/src/span_with_context.dart
+++ b/pkgs/source_span/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';