Report an error if a const constructor calls a non-const constructor.

tests/language/const_constructor_mixin_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 {
   const A(foo);
 }
 
-class B extends A with Mixin {
+class B extends A
+    with Mixin  /// 01: compile-time error
+    {
   const B(foo) : super(foo);
 }
 
 main() {
   var a = const B(42);
 }