Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/yaml/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:source_span/source_span.dart';
+
+/// A [SourceSpan] 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 SourceSpanMixin {
+  final SourceLocation start;
+  SourceLocation get end => start;
+  final text = "";
+
+  NullSpan(sourceUrl)
+      : start = new SourceLocation(0, sourceUrl: sourceUrl);
+}