Produce error when duplicate field initializers are found

tests/language/constructor_duplicate_initializers_test.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 initializers are not duplicated
 
 
 class Class {
   Class(var v)
     : field_ = v
       // Test against duplicate final field initializaion in initializing list.
     , field_ = 2  /// 01: compile-time error
   ;
   Class.field(this.field_)
       // Test against duplicate final field initialization between initializing
       // formals and initializer list.
     : field_ = 2  /// 02: compile-time error
   ;
+  // Test against duplicate final field initialization in initializing formals.
+  Class.two_fields(this.field_
+    , this.field_ /// 03: compile-time error
+  );
   final field_;
 }
 
 main() {
   new Class(42);
   new Class.field(42);
+  new Class.two_fields(42
+    , 42   /// 03: continued
+  );
 }