Use source_span rather than source_maps in csslib.

pkg/csslib/lib/css.dart

 // Copyright (c) 2013, 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 css;
 
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
-import 'package:source_maps/span.dart' show SourceFile;
+import 'package:source_span/source_span.dart';
 
 import 'parser.dart';
 import 'visitor.dart';
 import 'src/messages.dart';
 import 'src/options.dart';
 
 void main(List<String> arguments) {
   // TODO(jmesserly): fix this to return a proper exit code
   var options = PreprocessorOptions.parse(arguments);
   if (options == null) return;
 
   messages = new Messages(options: options);
 
   _time('Total time spent on ${options.inputFile}', () {
     _compile(options.inputFile, options.verbose);
   }, true);
 }
 
 void _compile(String inputPath, bool verbose) {
   var ext = path.extension(inputPath);
   if (ext != '.css' && ext != '.scss') {
     messages.error("Please provide a CSS/Sass file", null);
     return;
   }
   try {
     // Read the file.
     var filename = path.basename(inputPath);
     var contents = new File(inputPath).readAsStringSync();
-    var file = new SourceFile.text(inputPath, contents);
+    var file = new SourceFile(contents, url: path.toUri(inputPath));
 
     // Parse the CSS.
     var tree = _time('Parse $filename',
         () => new Parser(file, contents).parse(), verbose);
 
     _time('Analyzer $filename',
         () => new Analyzer([tree], messages), verbose).run();
 
     // Emit the processed CSS.
     var emitter = new CssPrinter();
@@ skipping 22 matching lines @@
 void _printMessage(String message, int duration) {
   var buf = new StringBuffer();
   buf.write(message);
   for (int i = message.length; i < 60; i++) buf.write(' ');
   buf.write(' -- ');
   if (duration < 10) buf.write(' ');
   if (duration < 100) buf.write(' ');
   buf..write(duration)..write(' ms');
   print(buf.toString());
 }