Use default values of named arguments when invoking the default super constructor.

tests/language/src/CompileTimeConstantLTest.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 A {
+  final x;
+  const A([x = 499]) : this.x = x;
+}
+
+class B extends A {
+  const B();
+  final z = 99;
+}
+
+class C extends B {
+  const C(this.y);
+  final y;
+}
+
+final v = const C(42);
+
+main() {
+  Expect.equals(42, v.y);
+  Expect.equals(499, v.x);
+  Expect.equals(99, v.z);
+}