Only support the new source_span spans in barback.

pkg/barback/lib/src/graph/package_graph.dart

Index: pkg/barback/lib/src/graph/package_graph.dart
diff --git a/pkg/barback/lib/src/graph/package_graph.dart b/pkg/barback/lib/src/graph/package_graph.dart
index e4664d9ca997058acf23a93651c1469127a04f08..0f9cde96d1576087b6bfce578481ad2c2e97f3e8 100644
--- a/pkg/barback/lib/src/graph/package_graph.dart
+++ b/pkg/barback/lib/src/graph/package_graph.dart
@@ -208,7 +208,7 @@ class PackageGraph {
       buffer.write("[${entry.level} ${entry.transform}] ");
 
       if (entry.span != null) {
-        buffer.write(entry.span.getLocationMessage(entry.message));
+        buffer.write(entry.span.message(entry.message));
       } else {
         buffer.write(entry.message);
       }