Check observer existence in domobserver methods

[ranmocy]

No description provided.

[marijnh]

I think flush has the same problem, and I'm wondering if it doesn't make more sense to remove all of these checks, given that ProseMirror isn't going to work anyway, on platforms that don't have mutationobserver. Did you run into this in a real situation? If so, which browser was that?

[ranmocy]

Updated for flush().

Not in production, but in tests with jsdom framework. It's undefined function issue since this.observer.takeRecords() is called but this.observer is undefined.

jsdom added the support of mutationobserver in jsdom/jsdom#639, and I'm waiting for next version of jest to include the version bump.

Even though it will eventually work, I still think it's better to do so for correctness. The observer should either be nullable in the caller, or silently no-op when methods are called.

[marijnh]

Thank you! Merged as f329f82