[web:tests] switch to new HTML DOM matcher

[yjbanov]

Remove the over-architected and under-utilized canonicalizeHtml and HtmlComparisonMode. Replace with a new hasHtml matcher.

The new matcher produces considerably better error messages when matching fails (see matchers_test.dart for multiple examples). It also allows focusing on particular parts of the DOM rather than looking for a full match. This way a test can test just the functionality it cares about, and it can skip specifying what unrelated parts should look like (i.e. it prevents over-testing). For example, let's say you want to test that an <flt-semantics> element has an aria-label attribute. You can use a DOM pattern that specified just that:

expect(
  element,
  hasHtml('<flt-semantics aria-label="hello"></flt-semantics>'),
)

This will check for the existence of <flt-semantics> and aria-label="hello". It will ignore everything else that's unrelated to ARIA labels. For example, the element may have its geometry specified in the style attribute, an id attribute, and many other things, but if the test doesn't care, it doesn't need to specify those things (it can, if needs to though). This also applies to style properties. See more examples in the matchers_test.dart file.

[mdebbar]

😱