Remove many things that depend on frog.

utils/import_mapper/passthrough_hook.dart

Index: utils/import_mapper/passthrough_hook.dart
===================================================================
--- utils/import_mapper/passthrough_hook.dart	(revision 6866)
+++ utils/import_mapper/passthrough_hook.dart	(working copy)
@@ -1,15 +0,0 @@
-// Copyright (c) 2012, 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 trivial identity import hook that generates an import map that maps all
- * imports to their original import string.
- */
-#library('passthrough_hook');
-
-#import('import_mapper.dart');
-
-void main() {
-  printImportMap((context, name) => name);
-}