[w3c/webcomponents] What's the expected behavior of a class with observedAttributes but no attributeChangedCallback? (#555)

I inadvertently created a component that defined `observedAttributes` without an `attributeChangedCallback` to handle the changes. Trying out this component (in the latest Custom Elements v1 polyfill) caused an error, which wasn't surprising, but it lead me to wonder: what *is* the standard behavior expected in that situation?

I couldn't infer what would happen from the spec: an exception at `define` time? Or should `observedAttributes` with no callback just be a no-op?

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