Now, everything goes through the extension methods in JSSymUtils.

Previously, we tested whether the default accessor itself had the
`@js.native` annotation. That is however never the case. Instead,
we now test whether the associated method has the annotation.

We still emit the *definitions* of those default accessors, even
though that they are never called anymore, because of backward
binary compatibility. If we stopped emitting them, a library
compiled with a newer Scala.js might not link anymore against an
application built with a previous Scala.js.

This is a forward port of
scala-js/scala-js@7e998b4

The default accessors matching that property are not emitted
anymore. This is similar to how default accessors for JS native
constructors are not emitted.

Unfortunately, for backward binary compatibility reasons, we must
still emit default accessors of JS native defs that are anything
else than `= js.native`. Until Scala.js 1.7.0, those default
accessors would actually be referenced by calling code (see
scala-js/scala-js#4554),
so removing them now would create dangling references.

This is a forward port of
scala-js/scala-js@e01dc1a

Nor for their default param accessors.

This is a forward port of
scala-js/scala-js@a95f3f6