Move generated files to gen/.

test/codegen/opassign.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.
-
-get index {
-  print('called "index" getter');
-  return 0;
-}
-
-final _foo = new Foo();
-get foo {
-  print('called "foo" getter');
-  return _foo;
-}
-
-class Foo { int x = 100; }
-
-main() {
-  var f = { 0: 40 };
-  print('should only call "index" 2 times:');
-  ++f[index];
-  forcePostfix(f[index]++);
-
-  print('should only call "foo" 2 times:');
-  ++foo.x;
-  forcePostfix(foo.x++);
-
-  print('op assign test, should only call "index" twice:');
-  f[index] += f[index];
-}
-
-// Postfix generates as prefix if the value isn't used. This method prevents it.
-forcePostfix(x) {}