Relax method override restrictions.

tests/language/method_override3_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.
+
+import 'package:expect/expect.dart';
+
+class A {
+  foo(required1, { named1: 499}) => -(required1 + named1 * 3);
+  bar(required1, required2, { named1: 13, named2: 17})
+      => -(required1 + required2 * 3 + named1 * 5 + named2 * 7);
+  gee({named1: 31}) => -named1;
+}
+
+class B extends A {
+  foo(required1) => required1;
+  bar(required1, required2, { named1: 29 })
+      => required1 + required2 * 3 + named1 * 5;
+  gee({named2: 11}) => named2 * 99;
+}
+
+main() {
+  var b = new B();
+  Expect.equals(499, b.foo(499));
+  Expect.equals(1 + 3 * 3 + 5 * 5, b.bar(1, 3, named1: 5));
+  Expect.equals(1 + 3 * 3 + 29 * 5, b.bar(1, 3));
+  Expect.equals(3 * 99, b.gee(named2: 3));
+  Expect.equals(11 * 99, b.gee());
+}