Fix class implementing FormatException getting warnings.

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 369 matching lines @@
     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)
       : super(message, span);
+
+  get source => null;

[nweiz, 2014/07/14 19:24:15]

This should be "span == null ? null : span.sourceUrl"

[nweiz, 2014/07/14 19:35:51]

I just looked at the source of FormatException and realized that this was meant
to be the source text, not the source location. In that case, this should be a
field that can be set via the constructor.

[Lasse Reichstein Nielsen, 2014/07/15 09:19:48]

Done. I have added the field, with an optional constructor argument, but I'll
leave it up to the users of SpanFormatException to figure out which source to
use.

+  int get position => null;

[nweiz, 2014/07/14 19:24:15]

This should be "span == null ? null : span.start.offset".

[Lasse Reichstein Nielsen, 2014/07/15 09:19:48]

Done.

 }