Support the latest source_span (dart-lang/source_maps#34)

diff --git a/pkgs/source_maps/pubspec.yaml b/pkgs/source_maps/pubspec.yaml
index 3e41803..0a35df6 100644
--- a/pkgs/source_maps/pubspec.yaml
+++ b/pkgs/source_maps/pubspec.yaml
@@ -12,5 +12,6 @@
   source_span: ^1.3.0
 
 dev_dependencies:
+  source_span: ^1.5.4
   test: ^1.2.0
   term_glyph: ^1.0.0