* Treat svg-wrapping spans differently from DOM-wrapping spans.
In preparation for the conversion of buildHTML to @flow, separate the two
different uses of domTree.span -- one to wrap a single SVG and one to wrap
arbitrary DOM nodes. This separation is to help avoid type issues that pop up
due to buildHTML methods accessing the .children field of spans as if they are
DOM elements and not SVG ones.
* Simplify the domTree DOM definitions based on separation of two span types.