Perform override and inheritance checks.

tests/language/check_method_override_test.dart

 // Copyright (c) 2013, 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 Mixin {
+class A {
+  f([var x]) {}
+  foo(var a, [x, y]) {}
 }
 
-class A {
-  const A(foo);
-}
-
-class B extends A
-    with Mixin  /// 01: compile-time error
-    {
-  const B(foo) : super(foo);
+class C extends A {
+  f() {} /// 01: static type warning
+  foo(var a, [x]) {} /// 02: static type warning
 }
 
 main() {
-  var a = const B(42);
-}
+  new A().foo(2);
+  new C().foo(1);
+}