Roll Observatory packages and add a roll script

packages/string_scanner/lib/src/exception.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 string_scanner.exception;
 
 import 'package:source_span/source_span.dart';
 
 /// An exception thrown by a [StringScanner] that failed to parse a string.
 class StringScannerException extends SourceSpanFormatException {
   String get source => super.source;
 
   /// The URL of the source file being parsed.
   ///
   /// This may be `null`, indicating that the source URL is unknown.
   Uri get sourceUrl => span.sourceUrl;
 
   StringScannerException(String message, SourceSpan span, String source)
       : super(message, span, source);
 }