Fix source spans for scalars.

lib/src/yaml_node.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.yaml_node;
 
 import 'dart:collection' as collection;
 
 import 'package:collection/collection.dart';
 import 'package:source_span/source_span.dart';
@@ skipping 148 matching lines @@
   ///
   /// [sourceUrl] may be either a [String], a [Uri], or `null`.
   YamlScalar.wrap(this.value, {sourceUrl})
       : style = ScalarStyle.ANY {
     _span = new NullSpan(sourceUrl);
   }
 
   /// Users of the library should not use this constructor.
   YamlScalar.internal(this.value, ScalarEvent scalar)
       : style = scalar.style {
-    _span = span;
+    _span = scalar.span;
   }
 
   /// Users of the library should not use this constructor.
   YamlScalar.internalWithSpan(this.value, SourceSpan span)
       : style = ScalarStyle.ANY {
     _span = span;
   }
 
   String toString() => value.toString();
 }
 
 /// Sets the source span of a [YamlNode].
 ///
 /// This method is not exposed publicly.
 void setSpan(YamlNode node, SourceSpan span) {
   node._span = span;
 }