Implement override checks.

dart/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();
+}