Summary:

Changelog: [internal]

This implements a (mostly) spec-compliant version of the [`Event`](https://dom.spec.whatwg.org/#interface-event) and [`EventTarget`](https://dom.spec.whatwg.org/#interface-eventtarget) Web interfaces.

It does not implement legacy methods in either of the interfaces, and ignores the parts of the spec that are related to Web-specific quirks (shadow roots, re-mapping of animation events with webkit prefixes, etc.).

IMPORTANT: This only creates the interfaces and does not expose them externally yet (no `Event` or `EventTarget` in the global scope).

Reviewed By: yungsters

Differential Revision: D67738145

Summary:

Changelog: [internal]

Creates a benchmarks to measure the performance of `EventTarget`.

Reviewed By: javache

Differential Revision: D67750677

Summary:

Changelog: [internal]

Adds a regression test to make sure we implement the correct spec-compliant behavior for a possible bug in ~~the Web spec~~ __Chrome__: whatwg/dom#1346

Edit: the bug is in the Chrome implementation, not in the spec.

Reviewed By: javache

Differential Revision: D67758702

…n of ReadOnlyNode (facebook#48427)

Summary:

Changelog: [internal]

The `ReactNativeElement` class was refactored for performance reasons, and the current implementation does **NOT** call `super()`, and it inlines the parent constructor instead.

When it eventually extends `EventTarget`, things won't work as expected because the existing `EventTarget` implementation has constructor dependencies.

This refactors the current implementation of `EventTarget` to eliminate those constructor side-effects, and eliminates the constructor altogether. 

This breaks encapsulation, but it has some positive side-effects on performance:
1. Creating `EventTarget` instances is faster because it has no constructor logic.
2. Improves memory by not creating maps to hold the event listeners if no event listeners are ever added to the target (which is very common).
3. Improves the overall runtime performance of the methods in the class by migrating away from private methods (which are known to be slow on the babel transpiled version we're currently using).

Extra: it also simplifies making window/the global scope implement the EventTarget interface :) 

## Benchmark results

Before:

| Latency average (ns) | Latency median (ns) | Samples | Task name                                              | Throughput average (ops/s) | Throughput median (ops/s) |
| ---|--- |--- |--- |---|---|
| 8234.22 ± 0.27%      | 8132.00             | 121445  | dispatchEvent, no bubbling, no listeners               | 122323 ± 0.02%             | 122971                   |
| 9001.22 ± 0.41%      | 8883.00             | 111097  | dispatchEvent, no bubbling, single listener            | 111981 ± 0.02%             | 112575                   |
| 51777.94 ± 0.58%     | 51247.00            | 19314   | dispatchEvent, no bubbling, multiple listeners         | 19393 ± 0.04%              | 19513                    |
| 8256.65 ± 0.29%      | 8152.00             | 121115  | dispatchEvent, bubbling, no listeners                  | 122031 ± 0.02%             | 122669                   |
| 9064.32 ± 0.44%      | 8933.00             | 110323  | dispatchEvent, bubbling, single listener per target    | 111265 ± 0.02%             | 111944                   |
| 51879.66 ± 0.27%     | 51447.00            | 19276   | dispatchEvent, bubbling, multiple listeners per target | 19325 ± 0.04%              | 19437               |

After:

| Latency average (ns) | Latency median (ns) | Samples | Task name                                              | Throughput average (ops/s) | Throughput median (ops/s)|
| ---------------------|---------------------|---------|--------------------------------------------------------|----------------------------|--------------------------|
| 5664.62 ± 0.50%      | 5588.00             | 176535  | dispatchEvent, no bubbling, no listeners               | 178219 ± 0.02%             | 178955                   |
| 7232.86 ± 0.50%      | 7131.00             | 138258  | dispatchEvent, no bubbling, single listener            | 139540 ± 0.02%             | 140233                   |
| 50957.51 ± 0.71%     | 50336.00            | 19625   | dispatchEvent, no bubbling, multiple listeners         | 19751 ± 0.04%              | 19866                    |
| 5692.36 ± 0.50%      | 5618.00             | 175675  | dispatchEvent, bubbling, no listeners                  | 177315 ± 0.02%             | 177999                   |
| 7277.82 ± 0.38%      | 7181.00             | 137404  | dispatchEvent, bubbling, single listener per target    | 138560 ± 0.02%             | 139256                   |
| 50493.64 ± 0.28%     | 50105.00            | 19805   | dispatchEvent, bubbling, multiple listeners per target | 19855 ± 0.04%              | 19958                    |

Reviewed By: yungsters

Differential Revision: D67758408

Summary:

Changelog: [internal]

This improves the performance of DOM `Event` interface implementation by migrating away from private fields.

Reviewed By: yungsters

Differential Revision: D67751821

Summary:

Changelog: [internal]

Makes the constants read-only and accessible through `Event.prototype` as well.

Reviewed By: yungsters

Differential Revision: D67830012