Fix class implementing FormatException getting warnings.

pkg/source_maps/lib/span.dart

Index: pkg/source_maps/lib/span.dart
diff --git a/pkg/source_maps/lib/span.dart b/pkg/source_maps/lib/span.dart
index 600506ee1e4323591ede8705eae5b9ece702c35f..771e9e0efa2804c58d93de49353902362d09fadc 100644
--- a/pkg/source_maps/lib/span.dart
+++ b/pkg/source_maps/lib/span.dart
@@ -387,4 +387,7 @@ class SpanException implements Exception {
 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.

 }