Make validate and invalidate protected

[sorvell]

These are useful extension points for scheduling rendering so they should be protected.

[Westbrook]

I'd love to help get this closed as it looks like a blocker to 1.0.0! If you aren't already working on it, of course...

This change is mainly updating:

private _validate() {

to

protected validate() {

and

private async _invalidate() {

to

protected async invalidate() {

and then making reference corrections across the repo, right?

I'll get started on putting together PR, but look forward to any other desires around this in the interim!

[Westbrook]

I could probably use some help with the README content, too!

[daKmoR]

As I just saw it here: https://github.com/Polymer/lit-element/pull/276/files#diff-c5f280d12b2d91f8b89fe4ac3b6313d8R522

I think it will still be

protected _validate() {
protected async _invalidate() {

to keep following the naming conventions (as real protected and private are probably still some time off for "plain" javascript 🙈 )

[Westbrook]

I guess it's a question of whether it's a lifecycle method, or an extension point... I was basing my decision off of https://github.com/Polymer/lit-element/blob/master/src/lib/updating-element.ts#L589

Hope this conversations is helping more than it's hurting @sorvell. Let me know which way I can move forward on this!