Revert "Implement override checks."

tests/language/bad_override_test.dart

-// Copyright (c) 2012, 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 Fisk {
-  get fisk() => null;
-  static /// 01: compile-time error
-  set fisk(x) {}
-  static /// 02: compile-time error
-  get hest() => null;
-  set hest(x) {}
-  foo() {}
-  var field;
-  method() {}
-  nullary() {}
-}
-
-class Hest extends Fisk {
-  static foo() {} /// 03: compile-time error
-  field() {} /// 04: compile-time error
-  var method; /// 05: compile-time error
-  nullary(x) {} /// 06: compile-time error
-}
-
-main() {
-  new Fisk();
-  new Hest();
-}