lib: let abort_controller target be EventTarget

The AbortController abort event should have EventTarget as a target
property of the argument event.

PR-URL: #35869
Backport-PR-URL: #38386
Refs: https://github.com/web-platform-tests/wpt/blob/master/dom/abort/event.any.js
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

lib/internal/abort_controller.js

@@ -9,6 +9,7 @@ const {
 } = primordials;
 
 const {
+  defineEventHandler,
   EventTarget,
   Event,
   kTrustEvent
@@ -52,21 +53,22 @@ function abortSignal(signal) {
   const event = new Event('abort', {
     [kTrustEvent]: true
   });
-  if (typeof signal.onabort === 'function') {
-    signal.onabort(event);
-  }
   signal.dispatchEvent(event);
 }
 
+// TODO(joyeecheung): V8 snapshot does not support instance member
+// initializers for now:
+// https://bugs.chromium.org/p/v8/issues/detail?id=10704
+const kSignal = Symbol('signal');
 class AbortController {
-  #signal = new AbortSignal();
-
   constructor() {
+    this[kSignal] = new AbortSignal();
     emitExperimentalWarning('AbortController');
+    defineEventHandler(this[kSignal], 'abort');
   }
 
-  get signal() { return this.#signal; }
-  abort() { abortSignal(this.#signal); }
+  get signal() { return this[kSignal]; }
+  abort() { abortSignal(this[kSignal]); }
 
   [customInspectSymbol](depth, options) {
     return customInspect(this, {

lib/internal/event_target.js

@@ -333,14 +333,13 @@ class EventTarget {
       }
       return event;
     };
+    if (event !== undefined)
+      event[kTarget] = this;
 
     const root = this[kEvents].get(type);
     if (root === undefined || root.next === undefined)
       return true;
 
-    if (event !== undefined)
-      event[kTarget] = this;
-
     let handler = root.next;
     let next;