d3-selection: Disable tslint error on TS 3.9 (#43269)
d3-selection: Disable tslint error on TS 3.9 (#43269)

DOM updates in 3.9 mean that Typescript now knows that
`HTMLBodyElement.ownerDocument` can't actually be null. Previously, a
non-null assertion was needed.

Because the tests have to compile on 3.9 AND older versions, I just
disabled the rule.
georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,markogresak/DefinitelyTyped,mcliment/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped