Use source_span rather than source_maps in csslib.

pkg/csslib/lib/src/validate.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.src.validate;
 
 import 'package:csslib/visitor.dart';
-import 'package:source_maps/span.dart' show Span;
+import 'package:source_span/source_span.dart';
 
 /** Can be thrown on any Css runtime problem includes source location. */
-class CssSelectorException implements Exception {
-  final String _message;
-  final Span _span;
-
-  CssSelectorException(this._message, [this._span]);
-
-  String toString() {
-    var msg = _span == null ? _message : _span.getLocationMessage(_message);
-    return 'CssSelectorException: $msg';
-  }
+class CssSelectorException extends SourceSpanException {
+  CssSelectorException(String message, [SourceSpan span])
+      : super(message, span);
 }
 
 List<String> classes = [];
 List<String> ids = [];
 
 class Validate {
   static int _classNameCheck(var selector, int matches) {
     if (selector.isCombinatorDescendant() ||
         (selector.isCombinatorNone() && matches == 0)) {
       if (matches < 0) {
@@ skipping 94 matching lines @@
       }
     }
 
     // Every selector must match.
     Selector selector = selectors[0];
     assert((matches >= 0 ? matches : -matches) ==
         selector.simpleSelectorSequences.length);
   }
 }