Move generated files to gen/.

test/codegen/temps.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.
-
-class FormalCollision {
-  // These shouldn't collide (see issue #136)
-  int _x, __x;
-  // This shouldn't generate a keyword as an identifier.
-  Function _function;
-  FormalCollision(this._x, this.__x, this._function);
-}
-
-class OptionalArg {
-  int opt, _opt;
-  OptionalArg([this._opt = 123]);
-  OptionalArg.named({this.opt: 456});
-}
-
-main() {
-  print(new FormalCollision(1, 2, (x) => x));
-  print(new OptionalArg()._opt);
-  print(new OptionalArg.named()._opt);
-}