Use source_span rather than source_maps in csslib.

pkg/csslib/lib/visitor.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // 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 csslib.visitor;
 
-import 'package:source_maps/span.dart' show Span;
+import 'package:source_span/source_span.dart';
 import 'parser.dart';
 
 part 'src/css_printer.dart';
 part 'src/tree.dart';
 part 'src/tree_base.dart';
 part 'src/tree_printer.dart';
 
 abstract class VisitorBase {
   void visitCssComment(CssComment node);
   void visitCommentDefinition(CommentDefinition node);
@@ skipping 428 matching lines @@
   void visitPaddingExpression(PaddingExpression node) {
     // TODO(terry): TBD
     throw UnimplementedError;
   }
 
   void visitWidthExpression(WidthExpression node) {
     // TODO(terry): TBD
     throw UnimplementedError;
   }
 }