Roll Observatory packages and add a roll script

packages/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: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);
 }