Move generated files to gen/.

test/codegen/extensions.dart

Index: test/codegen/extensions.dart
diff --git a/test/codegen/extensions.dart b/test/codegen/extensions.dart
deleted file mode 100644
index d31cf91ebb40db511abf732d14badecd9f256fe7..0000000000000000000000000000000000000000
--- a/test/codegen/extensions.dart
+++ /dev/null
@@ -1,11 +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.
-
-import 'dart:collection';
-
-class StringIterable extends IterableBase<String> {
-  final Iterator<String> iterator = null;
-}
-
-main() => new StringIterable();