Drop support for older Barback versions.

lib/src/asset/dart/serialize.dart

Index: lib/src/asset/dart/serialize.dart
diff --git a/lib/src/asset/dart/serialize.dart b/lib/src/asset/dart/serialize.dart
index f37ab70a179dd9004f8115800b379e68185676a9..3deb0ebc963f2606810fbdeb8e9bf836e625c409 100644
--- a/lib/src/asset/dart/serialize.dart
+++ b/lib/src/asset/dart/serialize.dart
@@ -9,10 +9,6 @@ import 'dart:isolate';
 
 import 'package:barback/barback.dart';
 
-//# if source_maps >=0.9.0 <0.10.0
-//> import 'package:source_maps/span.dart';
-//# end
-
 //# if source_span
 import 'package:source_span/source_span.dart';
 //# end
@@ -57,17 +53,6 @@ SourceSpan deserializeSpan(Map span) {
 ///
 /// [location] may be a [SourceLocation] or a [SourceLocation].
 Map serializeLocation(location) {
-//# if source_maps >=0.9.0 <0.10.0
-//>  if (location is Location) {
-//>    return {
-//>      'sourceUrl': location.sourceUrl,
-//>      'offset': location.offset,
-//>      'line': location.line,
-//>      'column': location.column
-//>    };
-//>  }
-//# end
-
 //# if source_span
   // TODO(nweiz): convert FileLocations to FileLocations.
   if (location is SourceLocation) {