Roll Observatory packages and add a roll script

packages/source_span/lib/src/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 source_span.span;
 
 import 'location.dart';
 import 'span_mixin.dart';
 
 /// A class that describes a segment of source text.
@@ skipping 58 matching lines @@
       throw new ArgumentError("Source URLs \"${start.sourceUrl}\" and "
           " \"${end.sourceUrl}\" don't match.");
     } else if (end.offset < start.offset) {
       throw new ArgumentError('End $end must come after start $start.');
     } else if (text.length != start.distance(end)) {
       throw new ArgumentError('Text "$text" must be ${start.distance(end)} '
           'characters long.');
     }
   }
 }