PostStream is a lazy module in 2.0, meaning it's only available when it's loaded, so you cannot import it in 'sync' mode (the normal import format)
When extending, you have to pass the module path as an argument instead, and remove the import line
ref: https://docs.flarum.org/2.x/extend/code-splitting#importing-split-modules-from-core-or-other-extensions
import { extend } from 'flarum/common/extend';
import app from 'flarum/forum/app';
extend('flarum/forum/components/PostStream', 'view', function (vdom) {}
when upgrading an extension from 1.0, this is automatically taken care of by the automated upgrade command: https://docs.flarum.org/2.x/extend/update-2_0#upgrade-automation
