Roll Observatory packages and add a roll script

packages/source_span/lib/src/span_with_context.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_with_context;
 
 import 'location.dart';
 import 'span.dart';
 import 'utils.dart';
 
@@ skipping 17 matching lines @@
       throw new ArgumentError(
           'The context line "$context" must contain "$text".');
     }
 
     if (findLineStart(context, text, start.column) == null) {
       throw new ArgumentError('The span text "$text" must start at '
           'column ${start.column + 1} in a line within "$context".');
     }
   }
 }