anchor.diff:  
     Iterable get keys {
       if (_isUpgraded) return _upgradedMap.keys;
  -    return _computeKeys().skip(0);
  +    return new _JsonMapKeyIterable(this);
     }
   
     Iterable get values {
  
positive.code:       => JS('=Object', 'Object.create(null)');
  }
  
  @patch class JsonDecoder {
    @patch
    StringConversionSink startChunkedConversion(Sink<Object> sink) {
  
negative.code: import 'dart:_js_helper' show patch;
  import 'dart:_foreign_helper' show JS;
  import 'dart:_interceptors' show JSExtendableArray;
  import 'dart:_internal' show MappedIterable, ListIterable;
  import 'dart:collection' show Maps, LinkedHashMap;
  
  /**
  