D3's Selection.node() returns an HTMLElement

d3_documentElement = document.documentElement and d3_select is
initialized from a querySelection so node() will only ever return
HTMLElement or null. We mistook it for SVGLocatable because of the
following in d3-test.js:

var box = label.node().getBBox();

But that's a different node() function altogether.
Merge pull request #1010 from flxf/d3selectionnode

D3's Selection.node() returns an HTMLElement