Only support the new source_span spans in barback.

pkg/barback/lib/src/log.dart

Index: pkg/barback/lib/src/log.dart
diff --git a/pkg/barback/lib/src/log.dart b/pkg/barback/lib/src/log.dart
index 691f4db48aef1727928162abc8b4aeef0201e1f0..e0d37b4df410e782d545ccfaac46be4b32c3fc20 100644
--- a/pkg/barback/lib/src/log.dart
+++ b/pkg/barback/lib/src/log.dart
@@ -4,8 +4,6 @@
 
 library barback.log;
 
-import 'package:source_maps/span.dart';
-
 import 'asset/asset_id.dart';
 import 'errors.dart';
 
@@ -35,7 +33,7 @@ class LogEntry {
 
   /// The location that the message pertains to or null if not associated with
   /// a source [Span].
-  final Span span;
+  final SourceSpan span;
 
   LogEntry(this.transform, this.assetId, this.level, this.message, this.span);
 }