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;
+  int get position => null;
 }