function filledBy( len, dtype, clbk ) {
	var buf;
	if ( dtype === 'complex64' ) {
		buf = filledarrayBy( len*2, 'float32', clbk );
		return new Complex64Array( buf.buffer );
	}
	if ( dtype === 'complex128' ) {
		buf = filledarrayBy( len*2, 'float64', clbk );
		return new Complex128Array( buf.buffer );
	}
	return filledarrayBy( len, dtype, clbk );
}
Returns a filled array according to a provided callback function.

@private
@param {NonNegativeInteger} len - array length
@param {string} dtype - array data type
@param {Function} clbk - callback
@returns {Collection} filled array