Allow initializing a field in a superclass in a const constructor.

tests/language/src/ConstConstructorTest.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 const constructor works with fields set in supertypes
+
+        
+class A {
+  final int a;
+  const A() : this.a = 1;
+}
+
+class B extends A {
+  const B(this.a);
+}
+
+int main () {
+  Expect.equals(1, const A().a);
+  Expect.equals(2, const B(2).a);
+}