[web:a11y] input.value test is a noop

[yjbanov]

Just noticed, while working on something else, that we have this line in our tests:

https://github.com/flutter/engine/blob/aeb987bce6ecf47e15de33ca7c0b0f85b6a0eb42/lib/web_ui/test/engine/semantics/semantics_test.dart#L1773

It is attempting to check that the <input> element has a value of "hello". However, because outerHTML strips away the value property, the test results in a false positive. The value is actually empty.

I'm not yet sure if this is a bug, but I'm filing this to leave a TODO and look into it later.