dillon1000/react

Enforce nodeCache validity

A node is considered valid if it

  1. has the expected ID (more of a sanity check than something that is
     ever likely to go wrong), and

  2. is contained by a currently mounted container.

When these requirements are met, we can be confident that
`ReactMount.findReactRenderedDOMNodeSlow(node.id) === node`, which is
important for cache consistency because `findReactRenderedDOMNodeSlow` is
what we fall back to when we don't find a node in the cache.

Point 2 is a subtle requirement, because it allows nodes to be valid even
if they are not currently contained by a document. Rendering into a
detached node is okay, in other words (which is something that
`document.getElementById` never properly accounted for).

Containment testing takes linear time in the depth of the DOM, which
sounds unfortunate until you realize that virtually all browsers support a
native `ancestor.contains(descendant)` method, and in practice the vast
majority of nodes are either orphaned with `.parentNode === null` or not
very deep relative to their container.