Unify splay tree iterators and interables constructor argument names.

[Mopriestt]

_SplayTreeKeyIterator has constructor argument _SplayTree<K, Node> map and _SplayTreeMapEntryIterator has constructor argument SplayTreeMap<K, V> tree. These two places don't follow the naming convention in rest of the file and could be misleading. Correcting them for better readability.

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[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/329721 has been updated with the latest commits from this pull request.