Change "FormatException.position" to be named "offset".

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 367 matching lines @@
 
   String toString({bool useColors: false, String color}) {
     if (span == null) return message;
     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)
+  final source;
+
+  SpanFormatException(String message, Span span, [this.source])
       : super(message, span);
 
-  get source => null;
-  int get position => null;
+  int get offset => span == null ? null : span.start.offset;
 }