Roll Observatory packages and add a roll script

packages/dart_style/lib/src/source_code.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 dart_style.src.source_code;
 
 /// Describes a chunk of source code that is to be formatted or has been
 /// formatted.
 class SourceCode {
   /// The [uri] where the source code is from.
@@ skipping 63 matching lines @@
       if (selectionLength < 0) {
         throw new ArgumentError("selectionLength must be non-negative.");
       }
 
       if (selectionStart + selectionLength > text.length) {
         throw new ArgumentError("selectionLength must end within text.");
       }
     }
   }
 }