anchor.diff:        }
       }
   
  -    if (isExtension!) {
  +    if (isExtension) {
         _ensureExtensions()
             ._setFieldAndInfo(fi as Extension<dynamic>, fieldValue);
       } else {
  
positive.code: 
    // Error-checking
  
    /// Checks the value for a field that's about to be set.
    void _validateField(FieldInfo fi, var newValue) {
      _ensureWritable();
      var message = _getFieldError(fi.type, newValue);
  
negative.code:     }
  
      if (fi.isRepeated) {
        if (_isGroupOrMessage(otherFi.type)) {
          // fieldValue must be a PbList of GeneratedMessage.
          PbList<GeneratedMessage> pbList = fieldValue;
          var repeatedFields = fi._ensureRepeatedField(meta, this);
  