Add new constructors to YamlNode subclasses.

pkg/yaml/lib/src/null_span.dart

+// Copyright (c) 2014, 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.null_span;
+
+import 'package:path/path.dart' as p;
+import 'package:source_maps/source_maps.dart';
+
+/// A [Span] with no location information.
+///
+/// This is used with [YamlMap.wrap] and [YamlList.wrap] to provide means of
+/// accessing a non-YAML map that behaves transparently like a map parsed from
+/// YAML.
+class NullSpan extends Span {
+  Location get end => start;
+  final text = "";
+
+  NullSpan(String sourceUrl)
+      : this._(new NullLocation(sourceUrl));
+
+  NullSpan._(Location location)
+      : super(location, location, false);
+
+  String formatLocationMessage(String message, {bool useColors: false,
+      String color}) {
+    var locationMessage = sourceUrl == null ? "in an unknown location" :
+        "in ${p.prettyUri(sourceUrl)}";
+    return "$locationMessage: $message";
+  }
+}
+
+/// A [Location] with no location information.
+///
+/// This is used with [YamlMap.wrap] and [YamlList.wrap] to provide means of
+/// accessing a non-YAML map that behaves transparently like a map parsed from
+/// YAML.
+class NullLocation extends Location {
+  final String sourceUrl;
+  final line = 0;
+  final column = 0;
+
+  String get formatString => sourceUrl == null ? "unknown location" : sourceUrl;
+
+  NullLocation(this.sourceUrl)
+      : super(0);
+}