Implement MutationObserver.disconnect()
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This implements https://dom.spec.whatwg.org/#dom-mutationobserver-disconnect
I added a `node_list` to the `MutationObserver` struct to keep track of the nodes involved in an observer because otherwise I could not find a way to unregister the observers on the node themselves without traversing the whole document tree. Let me know if there's something I missed.
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MutationObserver api: Implement takeRecords() and characterData mutations
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I started by looking at adding support for `takeRecords()` to get fluent.js to work on Servo. To get the wpt tests to pass I ended up implementing character data mutations (most of the plumbing was already there) and this fixed a few more tests.
We are still missing support for `disconnect()`, which I keep for a followup.
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Implement EventTarget constructor
Resolves#19283
Do "Wrap" functions only created for elements that aren't marked Abstract in .webidl file?
How can I see code that was generated from webidls?
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