Patch VS Code to wait for storage write (#2049)

VS Code has a short delay before writing storage (probably to queue up
rapid changes). In the web version of VS Code this happens on the client
which means if the page is reloaded before the delay expires the write
never happens.

Storage updates are already promises so this simply returns the promise
returned by the delayer so it won't resolve until the write actually
happens.

Fixes #2021.