Add check for duplicate field initialization

tests/language/compile_time_constant_e_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 = 3;
+  final x;
   final y;
   final z;
   final t;
 
-  const A([this.z = 99, tt = 100]) : y = 499, t = tt;
-  const A.named([z, this.t]) : y = 400 + z, this.z = z;
+  const A([this.z = 99, tt = 100]) : y = 499, t = tt, x = 3;
+  const A.named([z, this.t]) : y = 400 + z, this.z = z, x = 3;
   const A.named2([t, z, y, x]) : x = t, y = z, z = y, t = x;
 
   toString() => "A $x $y $z $t";
 }
 
 final a1 = const A(99, 100);
 final a2 = const A.named(99, 100);
 final a3 = const A.named2(1, 2, 3, 4);
 final a4 = const A();
 final a5 = const A(tt: 100, z: 99);
@@ skipping 38 matching lines @@
   Expect.equals("A null null null null", a8.toString());
 
   Expect.isTrue(a3 === a9);
 
   Expect.equals(4, a10.x);
   Expect.equals(3, a10.y);
   Expect.equals(2, a10.z);
   Expect.equals(1, a10.t);
   Expect.equals("A 4 3 2 1", a10.toString());
 }