Remove the checker and corresponding dead code

test/checker/dart_foobar.dart

Index: test/checker/dart_foobar.dart
diff --git a/test/checker/dart_foobar.dart b/test/checker/dart_foobar.dart
deleted file mode 100644
index 05866df839c37ec53860828b456ae257dde45e81..0000000000000000000000000000000000000000
--- a/test/checker/dart_foobar.dart
+++ /dev/null
@@ -1,12 +0,0 @@
-// Copyright (c) 2015, 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.
-
-/// This is a test for --url-mapping option.
-/// Analyzer's CustomUriResolver only supports on-disk resources, so we make
-/// this file accessible.
-library dart.foobar;
-
-class Baz {
-  static String quux = "hello world";
-}