Tree: Massive speedup in Tree#emptyNode().

Previously, `emptyNode()` triggered a node map reindex for each node
that was removed. This commit modifies `emptyNode()` to remove nodes in
last-first order and avoids the need to reindex.

Previously, emptying a node with 10,000 children took about 17 seconds
in Chrome. After this change, it's nearly instantaneous.
Build tree.
Merge branch 'dev-master' into dev-3.x

Conflicts:
	build/tree/tree-coverage.js
	build/tree/tree-min.js
	src/tree/HISTORY.md