Make YamlException inherit from SpanFormatException.

pkg/yaml/lib/src/yaml_exception.dart

Index: pkg/yaml/lib/src/yaml_exception.dart
diff --git a/pkg/yaml/lib/src/yaml_exception.dart b/pkg/yaml/lib/src/yaml_exception.dart
index a86327413d49fc9e05919881b5a1ac9d947266f5..9d437176834d42d4b54067e39f183f50cad7c44c 100644
--- a/pkg/yaml/lib/src/yaml_exception.dart
+++ b/pkg/yaml/lib/src/yaml_exception.dart
@@ -4,12 +4,11 @@
 
 library yaml.exception;
 
-/// An error thrown by the YAML processor.
-class YamlException implements Exception {
-  final String _msg;
-
-  YamlException(this._msg);
+import 'package:source_maps/source_maps.dart';
 
-  String toString() => _msg;
+/// An error thrown by the YAML processor.
+class YamlException extends SpanFormatException {

[Bob Nystrom, 2014/06/19 23:31:04]

I'm generally hesitant to inherit across package boundaries. Inheritance is just
about the tightest coupling possible, so it can set you up for pain.

Would it work to have YamlException contain a SpanFormatException?

[nweiz, 2014/06/19 23:47:52]

The point of SpanException and SpanFormatException is for other packages to
inherit them. For example, it's important that applications (such as Pub) can do

    if (error is SpanException) {
      print(error.toString(useColor: true))
    }

in their top-level error handling.

+  YamlException(String message, Span span)
+      : super(message, span);
 }