Fix a bug in FixedSpan.getLocationMessage.

pkg/source_maps/lib/span.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.
 
 /// Dart classes representing the souce spans and source files.
 library source_maps.span;
 
 import 'dart:math' show min, max;
 
 import 'package:path/path.dart' as p;
@@ skipping 46 matching lines @@
     int d = start.compareTo(other.start);
     return d == 0 ? end.compareTo(other.end) : d;
   }
 
   /// Gets the location in standard printed form `filename:line:column`, where
   /// line and column are adjusted by 1 to match the convention in editors.
   String get formatLocation => start.formatString;
 
   String getLocationMessage(String message,
       {bool useColors: false, String color}) {
-    var source = url == null ? '' : ' of ${p.prettyUri(url)}';
+    var source = start.sourceUrl == null ? '' :
+        ' of ${p.prettyUri(start.sourceUrl)}';
     return 'line ${start.line + 1}, column ${start.column + 1}$source: ' +
         message;
   }
 
   bool operator ==(Span other) =>
     sourceUrl == other.sourceUrl && start == other.start && end == other.end;
 
   int get hashCode => sourceUrl.hashCode + start.offset + (31 * length);
 
   String toString() => '<$runtimeType: $start $end $formatLocation $text>';
@@ skipping 302 matching lines @@
     return "Error on " + span.getLocationMessage(message,
         useColors: useColors, color: color);
   }
 }
 
 /// A [SpanException] that's also a [FormatException].
 class SpanFormatException extends SpanException implements FormatException {
   SpanFormatException(String message, Span span)
       : super(message, span);
 }