function returnTypeEnvVar( dt ) {
	return 'JAVASCRIPT_RETURN_TYPE=\'' + dt + '\'';
}
Returns a `JAVASCRIPT_RETURN_TYPE` environment variable string.

@private
@param {string} dt - return type
@returns {string} environment variable string

@example
var s = returnTypeEnvVar( 'number' );
// returns 'JAVASCRIPT_RETURN_TYPE=\'number\''