Relax method override restrictions.

tests/language/method_override4_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.throws(() => b.foo(499, named1: 88), (e) => e is NoSuchMethodError);
+  Expect.throws(() => b.bar(1, 2, named1: 3, named2: 88),
+                (e) => e is NoSuchMethodError);
+  Expect.throws(() => b.bar(1, 2, named2: 88),
+                (e) => e is NoSuchMethodError);
+  Expect.throws(() => b.gee(named1: 3),
+                (e) => e is NoSuchMethodError);
+}