Using [[ArrayBufferData]] for shared memory makes SharedArrayBuffers transferable, contrary to intent #144

According to the HTML spec, if an object has an [[ArrayBufferData]] internal slot that is not detached then the object is transferable, see https://html.spec.whatwg.org/multipage/infrastructure.html#istransferable.

Recently we made the change that we use [[ArrayBufferData]] even for a Shared Data Block, and this slot cannot become detached. This was a much-desired change. But roughly at the same time we decided that postMessage() should not allow for a SharedArrayBuffer in the transfer list, and this is now encoded in the SAB companion spec for DOM, see https://tc39.github.io/ecmascript_sharedmem/dom_shmem.html.

Clearly these decisions are not quite compatible. The fix is probably to hack up the HTML spec to discriminate better. This needs to be noted with greater specificity in section 1.5 of our DOM companion spec.