Setup correct scope for initializers and complain about fields initialized more than once.

pkg/front_end/lib/src/fasta/builder/formal_parameter_builder.dart

 // Copyright (c) 2016, 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.
 
 library fasta.formal_parameter_builder;
 
 import '../parser/parser.dart' show FormalParameterType;
 
 import 'builder.dart'
     show LibraryBuilder, MetadataBuilder, ModifierBuilder, TypeBuilder;
@@ skipping 25 matching lines @@
   bool get isPositional => kind.isPositional || kind.isRequired;
 
   bool get isNamed => kind.isNamed;
 
   bool get isOptional => !isRequired;
 
   bool get isLocal => true;
 
   @override
   String get fullNameForErrors => name;
+
+  FormalParameterBuilder forFormalParameterInitializerScope();
 }