Add createIFrameElement helper

[kenzieschmoll]

In trying to migrate from dart:html to package:web, it was not obvious to me how to create an iFrame element. Not sure if you think this is special enough to special case with its own helper, but thought I'd push up a PR to propose.

@srujzs

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:web	0.3.1-dev	ready to publish	v0.3.1-dev

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[kevmoo]

Please add a changelog entry!

[srujzs]

I'm okay with adding helpers for these as needed, since it's kind of a downgrade from dart:html to create elements with string values.

Eventually, we might want to have a helper for each element type and put them in a separate file, but for now, LGTM!