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

[ahe, 2012/08/16 11:56:43]

The test is more precise if you write it like this:

  Class.two_fields(this.field_
                  // Test against duplicate final field initialization
                  // in initializing formals.
                  , this.field_ /// 03: compile-time error
                  );

The advantage is that it reduces the chance that there is something else wrong
on the line and we end up not testing what we think we're testing.

   final field_;
 }
 
 main() {
   new Class(42);
   new Class.field(42);
+  new Class.two_fields(42, 42); /// 03: continued

[ahe, 2012/08/16 11:56:43]

Then this would also need rewriting.

 }