Support ignoring unknown tags

[swankjesse]

Suppose we encounter an unknown WidgetTag. I’d like to suppress all future ProtocolMismatchHandler updates on that tag, including those on its properties and its children. The unknown widget should not adversely impact the indexing of its sibling widgets.

Similarly for unknown ChildrenTag values: I’d like one ProtocolMismatchHandler to signal the problem, and then no further updates.

[JakeWharton]

The impact of this will be quite large, because we effectively have to create tombstone widgets that somehow make it into the children collections but not the native UI tree.

[JakeWharton]

Decisions:

• Properties: do nothing. The callback can dedupe if it wants because it gets widget tag + property tag.

• Children: I think we can just return null? Nodes will get added to the map, and any children ops will get ignored. Tests to confirm. The callback can dedupe if it wants because it gets widget tag + children tag.

• Widget: Introduce NullWidget or something. This holds the position to ensure indices on future operations remain correct. Ideally it's a GONE view/display:none; DOM node/empty composable/whatever on iOS and we don't have to special case it. Tests to confirm.

[JakeWharton]

A null widget should NOT notify the listener for missing properties or children!