Use SpanScanner to emit better YAML parse errors.

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 'src/composer.dart';
 import 'src/constructor.dart';
 import 'src/parser.dart';
 import 'src/yaml_exception.dart';
 
 export 'src/yaml_exception.dart';
 export 'src/yaml_map.dart';
 
 /// Loads a single document from a YAML string.
 ///
 /// If the string contains more than one document, this throws a
 /// [YamlException]. In future releases, this will become an [ArgumentError].
 ///
 /// 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.
-loadYaml(String yaml) {
-  var stream = loadYamlStream(yaml);
+///
+/// If [sourceName] is passed, it's used as the name of the file or URL from
+/// which the YAML originated for error reporting.
+loadYaml(String yaml, {String sourceName}) {
+  var stream = loadYamlStream(yaml, sourceName: sourceName);
   if (stream.length != 1) {
     throw new YamlException("Expected 1 document, were ${stream.length}.");
   }
   return stream[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.
-List loadYamlStream(String yaml) {
-  return new Parser(yaml).l_yamlStream()
+///
+/// If [sourceName] is passed, it's used as the name of the file or URL from
+/// which the YAML originated for error reporting.
+List loadYamlStream(String yaml, {String sourceName}) {
+  var stream;
+  try {
+    stream = new Parser(yaml, sourceName).l_yamlStream();
+  } on FormatException catch (error) {
+    throw new YamlException(error.toString());
+  }
+
+  return stream
       .map((doc) => new Constructor(new Composer(doc).compose()).construct())
       .toList();
 }