Fixed issues due to Y.DOM being undefined when event-base was attached in rare use cases

Fixes #2533242

The `YDOM = Y.DOM` alias we added in 3.7.3 would end up being undefined
in certain custom module dependency use cases, which would result in
`nodelist.on()` failing to attach listners.

In order to bring out the issue, all the following need to apply:

1. Custom modules with deeper node-base dependency.
2. node-base repeated in `use` statement.
3. Module dependencies for custom modules not provided to YUI() instance config (that is, not available to loader).

This is not the root fix. The root fix is probably in loader somewhere, but given the catch-22 complexity between node-base and event-base attach order, and the rarity of the use case, this seemed like the better, less fragile way to go.

Confirmed that the unit test added passes with this commit, and fails starting with 3.7.3, when this issue was introduced.

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