frog: check that const constructors have potentially const initializers. Also

tests/language/src/ConstInit6NegativeTest.dart

+// Copyright (c) 2011, 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.
+// Check that initializer is expected after final variable declaration.
+
+class ConstInit6NegativeTest {
+  final x;
+  const ConstInit6NegativeTest()
+    : x = 1.toString();  // Error: not a compile time const.
+}
+
+main() {
+  const ConstInit6NegativeTest();
+}