message: use is instead of as in Button RIP
diff: @@ -12,7 +12,7 @@ injectGlobal(sass`
  @import "primer-buttons/index.scss";
  `)
  -function proto({children, size, grouped, scheme, onClick, disabled, className, ...rest}) {
  +function proto({is: Tag, children, size, grouped, scheme, onClick, disabled, className, ...rest}) {
  const classes = classnames(
  className,
  'btn',
  @@ -25,9 +25,9 @@ function proto({children, size, grouped, scheme, onClick, disabled, className, .
  )
  return (
  - <button {...rest} type="button" disabled={disabled} onClick={disabled ? undefined : onClick} className={classes}>
  + <Tag {...rest} type="button" disabled={disabled} onClick={disabled ? undefined : onClick} className={classes}>
  {children}
  - </button>
  + </Tag>
  )
  }
  @@ -39,6 +39,7 @@ const Button = styled(proto)`
  Button.defaultProps = {theme}
  Button.propTypes = {
  + is: PropTypes.oneOfType([PropTypes.oneOf(['button', 'a', 'summary', 'input']), PropTypes.func]),
  children: PropTypes.node,
  disabled: PropTypes.bool,
  grouped: PropTypes.bool,
  