Previously this was compiled to require an object despite the IDL file
specifying 'optional'.
This commit makes IDLGenerator respect this modifier, and fixes the only
affected instance.
Although we translate e.g `block` to `block flow` for internal use in
the engine, CSS-DISPLAY-3 tells us to use the short form in
serializations for compatibility reasons.
This adds 9 points to our score on https://html5test.com/ :^)
This allows us to create "text tests" in addition to "layout tests".
Text tests work the same as layout tests, but dump the document content
as text and exit upon receiving the window "load" event.
