Move generated files to gen/.

test/codegen/names.dart

-// 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.
-
-var exports = 42;
-
-class Foo {
-  _foo() => 123;
-}
-
-_foo() => 456;
-
-class Frame {
-  final List arguments;
-  Frame.caller(this.arguments);
-  static callee() => null;
-}
-
-
-class Frame2 {
-  static int caller = 100;
-  static int arguments = 200;
-}
-
-main() {
-  print(exports);
-  print(new Foo()._foo());
-  print(_foo());
-  print(new Frame.caller([1,2,3]));
-  var eval = Frame.callee;
-  print(eval);
-  print(Frame2.caller + Frame2.arguments);
-}