in lib/presence-stack.ts [122:129]
private createResolver(typeName: string, fieldName: string, options: ResolverOptions)
: AppSync.BaseDataSource {
let source = (typeof(options.source) === 'string') ?
this.api.addLambdaDataSource(`${options.source}DS`, this.getFn(options.source)) :
options.source;
source.createResolver({ typeName, fieldName, ...options });
return source;
};