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

pkg/front_end/lib/src/fasta/builder/field_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.field_builder;
 
 import 'builder.dart' show LibraryBuilder, MemberBuilder;
 
 import 'package:kernel/ast.dart' show DartType;
 
 abstract class FieldBuilder<T> extends MemberBuilder {
   final String name;
 
   final int modifiers;
 
   FieldBuilder(
       this.name, this.modifiers, LibraryBuilder compilationUnit, int charOffset)
       : super(compilationUnit, charOffset);
 
   DartType get builtType;
 
   void set initializer(T value);
 
+  bool get hasInitializer;
+
   bool get isField => true;
 }