Delete a bunch of packages that are now on GitHub.

pkg/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);
-}