Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/string_scanner/string_scanner.dart

Index: observatory_pub_packages/string_scanner/string_scanner.dart
===================================================================
--- observatory_pub_packages/string_scanner/string_scanner.dart	(revision 0)
+++ observatory_pub_packages/string_scanner/string_scanner.dart	(working copy)
@@ -0,0 +1,11 @@
+// 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.
+
+/// A library for parsing strings using a sequence of patterns.
+library string_scanner;
+
+export 'src/exception.dart';
+export 'src/line_scanner.dart';
+export 'src/span_scanner.dart';
+export 'src/string_scanner.dart';