Roll Observatory packages and add a roll script

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);
-}