Make YamlException inherit from SpanFormatException.

pkg/yaml/lib/yaml.dart

 // Copyright (c) 2012, 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.
 
 library yaml;
 
+import 'package:string_scanner/string_scanner.dart';
+
 import 'src/composer.dart';
 import 'src/constructor.dart';
 import 'src/parser.dart';
 import 'src/yaml_exception.dart';
 import 'src/yaml_node.dart';
 
 export 'src/yaml_exception.dart';
 export 'src/yaml_node.dart';
 
 /// Loads a single document from a YAML string.
@@ skipping 16 matching lines @@
     loadYamlNode(yaml, sourceName: sourceName).value;
 
 /// Loads a single document from a YAML string as a [YamlNode].
 ///
 /// This is just like [loadYaml], except that where [loadYaml] would return a
 /// normal Dart value this returns a [YamlNode] instead. This allows the caller
 /// to be confident that the return value will always be a [YamlNode].
 YamlNode loadYamlNode(String yaml, {String sourceName}) {
   var stream = loadYamlStream(yaml, sourceName: sourceName);
   if (stream.length != 1) {
-    throw new YamlException("Expected 1 document, were ${stream.length}.");
+    throw new YamlException("Expected 1 document, were ${stream.length}.",
+        stream.span);
   }
   return stream.nodes[0];
 }
 
 /// Loads a stream of documents from a YAML string.
 ///
 /// The return value is mostly normal Dart objects. However, since YAML mappings
 /// support some key types that the default Dart map implementation doesn't
 /// (NaN, lists, and maps), all maps in the returned document are [YamlMap]s.
 /// These have a few small behavioral differences from the default Map
 /// implementation; for details, see the [YamlMap] class.
 ///
 /// In future versions, maps will instead be [HashMap]s with a custom equality
 /// operation.
 ///
 /// If [sourceName] is passed, it's used as the name of the file or URL from
 /// which the YAML originated for error reporting.
 YamlList loadYamlStream(String yaml, {String sourceName}) {
   var pair;
   try {
     pair = new Parser(yaml, sourceName).l_yamlStream();
-  } on FormatException catch (error) {
-    throw new YamlException(error.toString());
+  } on StringScannerException catch (error) {
+    throw new YamlException(error.message, error.span);
   }
 
   var nodes = pair.first
       .map((doc) => new Constructor(new Composer(doc).compose()).construct())
       .toList();
   return new YamlList(nodes, pair.last);
 }