This example is slightly bogus.
If we could do so, we would improve on the JS DOM and declare TextInputElement.value as a non-nullable String.
To see this, consider it is not possible to load the value fields and get something that is not a String. Converting null and undefined are in the same category of crazy JavaScript conversions as converting numbers or objects with a valueOf field.

I am worried about the code size and performance implications of putting conversions on DOM APIs.
It will prevent inlining and cause setters to be encoded as functions rather that fields. We will never match JS performance if we have to insert checks.

Browsers are not totally consistent in their implementation of the standards.

Why do you think the example is bogus? Just consider a setting where you use some framework with data binding that binds a List of String to a set of input fields. If you push the initial state of your List to the input fields, you would get loads of ¨undefined¨ unless your framework has some special handling to work around this quirk.

I would also be OK with disallowing null in the first place.

Set owner to @rakudrama.

Issue #21684 has been merged into this issue.

The example given is totally valid. I hit this exact issue in a real project I was prototyping (see Issue #21684). Everything worked fine in Dartium; then when I compiled to JS all of my forms were full of "undefined" strings.

Whether assigning null to a textboxes value is a good thing to do or not; the more differences there are between Dartium and "real" browsers, the more frustrating and expensive development with Dart will be. We're starting to see that we just can't trust how things work in Dartium; and that's going to be a serious barrier to adoption.

It seems reasonable that we could force this to be an error in Dartium without impacting dart2js performance.

How?
Dartium doesn't have any null/undefined distinction. It can't warn you when you pass an undefined instead of a null to the DOM.

I meant to add null-checks in Dartium for these sorts of APIs. So rather than just converting null to an empty string it would throw.