Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/yaml/src/yaml_exception.dart

Index: observatory_pub_packages/yaml/src/yaml_exception.dart
===================================================================
--- observatory_pub_packages/yaml/src/yaml_exception.dart	(revision 0)
+++ observatory_pub_packages/yaml/src/yaml_exception.dart	(working copy)
@@ -0,0 +1,14 @@
+// Copyright (c) 2013, 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.exception;
+
+import 'package:source_span/source_span.dart';
+
+/// An error thrown by the YAML processor.
+class YamlException extends SourceSpanFormatException {
+  YamlException(String message, SourceSpan span)
+      : super(message, span);
+}
+