addExtension method
      
void
addExtension(
    
- Extension extension,
- dynamic value
inherited
    Adds an extension field value to a repeated field.
The backing List will be created if necessary. If the list already exists, the old extension won't be overwritten.
Implementation
void addExtension(Extension extension, var value) {
  if (!extension.isRepeated) {
    throw ArgumentError(
        'Cannot add to a non-repeated field (use setExtension())');
  }
  _fieldSet._ensureExtensions()._ensureRepeatedField(extension).add(value);
}