Move generated files to gen/.

test/codegen/corelib/string_operations_with_null_test.dart~

-// Copyright (c) 2013, 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 "package:expect/expect.dart";
-import "../language/compiler_annotations.dart";
-
-@DontInline()
-returnStringOrNull() {
-  () => 42;
-  return new DateTime.now().millisecondsSinceEpoch == 0 ? 'foo' : null;
-}
-
-main() {
-  Expect.throws(() => 'foo' + returnStringOrNull(),
-                (e) => e is ArgumentError);
-  Expect.throws(() => 'foo'.split(returnStringOrNull()),
-                (e) => e is ArgumentError || e is NoSuchMethodError);
-}