Best way to track disposable things in Node

[samreid]

While working on #1047 and #490, @zepumph and I discussed this TODO in Node.js that says:

        // TODO: Should this be set to null in the constructor, https://github.com/phetsims/scenery/issues/490
        const phetioVisibleProperty = new BooleanProperty( this.visible, merge( {

This is later set on the Node for disposal purposes:

        // When created in this manner, the phetioVisibleProperty is "owned" by this Node and we are responsible
        // for its disposal. Assign after calling this.visibleProperty = ... since it disposes this.phetioVisibleProperty
        // if it exists
        this.phetioVisibleProperty = phetioVisibleProperty;

Should we set this.phetioVisibileProperty=null in the constructor? Should we add a Node.disposeEmitter that we append items to?

[zepumph]

After discussing with @jonathanolson, we felt like it was best to include a declaration in the Node constructor set to null.