anchor.diff:    return ctx;
   }
   
  -class _ShimField extends FieldElementImpl {
  +/// A manually-instantiated [FieldElement].
  +class ShimFieldImpl extends FieldElementImpl {
     @override
     final DartType type;
  -  _ShimField(String name, this.type) : super(name, -1);
  +  ShimFieldImpl(String name, this.type) : super(name, -1);
   }
  
positive.code:           dateTime = dt.type;
          }
  
          var field = new _ShimField(key, dateTime);
          ctx.aliases[key] = new ReCase(key).snakeCase;
          ctx.fields.add(field);
          ctx.shimmed[key] = true;
  
negative.code: 
    if (autoIdAndDateFields != false) {
      if (!fieldNames.contains('id')) {
        var idField = new ShimFieldImpl('id', lib.context.typeProvider.stringType);
        ctx.fields.insert(0, idField);
        ctx.shimmed['id'] = true;
      }
  