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

pkg/front_end/lib/src/fasta/kernel/kernel_library_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.kernel_library_builder;
 
 import 'package:front_end/src/scanner/token.dart' show Token;
 
 import 'package:kernel/ast.dart';
 
@@ skipping 445 matching lines @@
     KernelNamedTypeBuilder supertype = applyMixins(mixinApplication,
         metadata: metadata,
         name: name,
         typeVariables: typeVariables,
         modifiers: modifiers,
         interfaces: interfaces,
         charOffset: charOffset);
     checkTypeVariables(typeVariables, supertype.builder);
   }
 
+  @override
   void addField(List<MetadataBuilder> metadata, int modifiers,
       KernelTypeBuilder type, String name, int charOffset, Token initializer) {
     addBuilder(
         name,
         new KernelFieldBuilder(
             metadata, type, name, modifiers, this, charOffset, initializer),
         charOffset);
   }
 
   String computeAndValidateConstructorName(String name, int charOffset) {
@@ skipping 371 matching lines @@
           mixinApplicationClasses.putIfAbsent(name, () => builder);
       if (existing != builder) {
         part.scope.local.remove(name);
       }
     });
     super.includePart(part);
     nativeMethods.addAll(part.nativeMethods);
     boundlessTypeVariables.addAll(part.boundlessTypeVariables);
   }
 }