hide internal props

[milahu]

old:

> JSON.stringify(decl, 0, 2)
{
  _values: { .... },
  _importants: { .... },
  _length: 1234,
  _onChange: function() {},
  '0': 'display',
  '1': 'color'
}

new:

> JSON.stringify(decl, 0, 2)
{
  '0': 'display',
  '1': 'color'
}