It looks like these classes are a legacy from version 1 but we actually never promoted their usage. Instead, the regular way to create a chart is to set the type in the config, for example: `new Chart(ctx, {type: 'bar'})`. Some types are actually missing (no `Chart.HorizontalBar` or `Chart.Pie`) but it's also not scalable because it can easily conflict with other classes scoped under the `Chart` namespace.