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

tests/language/const_constructor_mixin2_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 {
+  var nonFinalField;
+}
+
+class A {
+  const A(foo);
+}
+
+class B extends A 
+    with Mixin  /// 01: compile-time error
+    {
+  const B(foo) : super(foo);
+}
+
+main() {
+  var a = const B(42);
+  a.nonFinalField = 54; /// 01: continued
+}