Perform override and inheritance checks.

tests/language/check_method_override_test.dart

Index: tests/language/check_method_override_test.dart
diff --git a/tests/language/const_constructor_mixin_test.dart b/tests/language/check_method_override_test.dart
similarity index 56%
copy from tests/language/const_constructor_mixin_test.dart
copy to tests/language/check_method_override_test.dart
index 83ad1563635e0a7a8477bc813f37784760702546..1fabfdd5b5a9d09a1dde2a82c2c5bf1951ef0428 100644
--- a/tests/language/const_constructor_mixin_test.dart
+++ b/tests/language/check_method_override_test.dart
@@ -2,19 +2,17 @@
 // 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 {
-  const A(foo);
+  f([var x]) {}
+  foo(var a, [x, y]) {}
 }
 
-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);
+}