Add check for duplicate field initialization

tests/language/compile_time_constant_g_test.dart

 // Copyright (c) 2012, 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 A {
-  final x = 4;
+  final x;
   const A(this.x);
   const A.named([this.x]);
   const A.named2([this.x = 2]);
-  const A.named3();
 }
 
 class B extends A {
   const B(x) : super(x + 10);
   const B.named_() : super.named();
   const B.named(x) : super.named(x + 10);
   const B.named2_() : super.named2();
   const B.named2(x) : super.named2(x + 10);
-  const B.named3() : super.named3();
 }
 
 final b1 = const B(0);
 final b2 = const B.named_();
 final b3 = const B.named(1);
 final b4 = const B.named2_();
 final b5 = const B.named2(3);
-final b6 = const B.named3();
 
 main() {
   Expect.equals(10, b1.x);
   Expect.equals(null, b2.x);
   Expect.equals(11, b3.x);
   Expect.equals(2, b4.x);
   Expect.equals(13, b5.x);
-  Expect.equals(4, b6.x);
 }