Add check for constness in constructor redirection

tests/language/regress_27617_test.dart

 // Copyright (c) 2016, 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 Foo {
-  static int x = 42;
+  final String greeting;
+  Foo._(this.greeting) { }
+
+  // Const constructor must not redirect to non-const constructor.
+  const Foo.hi() : this._('hi');  /// 1: compile-time error
 }
 
-class Bar extends Foo {
-  static int x = 12;
+main() {
+  const h = const Foo.hi();  /// 1: continued
 }
-
-void main() {
-  Expect.equals(12, Bar.x);
-}