bound element references arent triggering subscriptions when changing to null

[halfnelson]

https://v3.svelte.technology/repl?version=3.0.0-alpha25&gist=da9c704874c5bfed8827c14d35c8e3b4

Steps to Reproduce:

• Bind an element to a variable
• use that variable in another attribute binding
• wrap the bound element in an if block that will remove it from the dom
• trigger the false condition for the if block and observe that the attribute binding referencing the variable is not updated.

Expected Behaviour?:
When an element goes out of scope its binding variable is set to null, any reactive subscriptions to that variable will be updated including template expressions and $ expressions.

Actual Behaviour:
Neither the template binding or a reactive $ binding are updated, the value of the reference is null when logged to console though.

[Rich-Harris]

Fixed in 3.6.3, thanks