Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/string_scanner/src/exception.dart

Index: observatory_pub_packages/string_scanner/src/exception.dart
===================================================================
--- observatory_pub_packages/string_scanner/src/exception.dart	(revision 0)
+++ observatory_pub_packages/string_scanner/src/exception.dart	(working copy)
@@ -0,0 +1,20 @@
+// 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);
+}