lib: define Event.isTrusted in the prototype

lib/internal/event_target.js

@@ -119,8 +119,6 @@ class Event {
       isTrustedSet.add(this);
     }
 
-    // isTrusted is special (LegacyUnforgeable)
-    ObjectDefineProperty(this, 'isTrusted', isTrustedDescriptor);
     this[kTarget] = null;
     this[kIsBeingDispatched] = false;
   }
@@ -343,6 +341,11 @@ ObjectDefineProperties(
     eventPhase: kEnumerableProperty,
     cancelBubble: kEnumerableProperty,
     stopPropagation: kEnumerableProperty,
+    // Don't conform to the spec with isTrusted. The spec defines it as
+    // LegacyUnforgeable but defining it in the constructor has a big
+    // performance impact and the property doesn't seem to be useful outside of
+    // browsers.
+    isTrusted: isTrustedDescriptor,
   });
 
 function isCustomEvent(value) {

test/parallel/test-abortcontroller.js

@@ -57,9 +57,9 @@ const { setTimeout: sleep } = require('timers/promises');
   }));
   first.abort();
   second.abort();
-  const firstTrusted = Reflect.getOwnPropertyDescriptor(ev1, 'isTrusted').get;
-  const secondTrusted = Reflect.getOwnPropertyDescriptor(ev2, 'isTrusted').get;
-  const untrusted = Reflect.getOwnPropertyDescriptor(ev3, 'isTrusted').get;
+  const firstTrusted = Reflect.getOwnPropertyDescriptor(Object.getPrototypeOf(ev1), 'isTrusted').get;
+  const secondTrusted = Reflect.getOwnPropertyDescriptor(Object.getPrototypeOf(ev2), 'isTrusted').get;
+  const untrusted = Reflect.getOwnPropertyDescriptor(Object.getPrototypeOf(ev3), 'isTrusted').get;
   strictEqual(firstTrusted, secondTrusted);
   strictEqual(untrusted, firstTrusted);
 }

test/parallel/test-events-customevent.js

@@ -187,7 +187,7 @@ const { Event, EventTarget, CustomEvent } = require('internal/event_target');
 }
 {
   const ev = new CustomEvent('foo');
-  deepStrictEqual(Object.keys(ev), ['isTrusted']);
+  strictEqual(ev.isTrusted, false);
 }
 
 // Works with EventTarget

test/parallel/test-eventtarget.js

@@ -126,7 +126,7 @@ let asyncTest = Promise.resolve();
 }
 {
   const ev = new Event('foo');
-  deepStrictEqual(Object.keys(ev), ['isTrusted']);
+  strictEqual(ev.isTrusted, false);
 }
 {
   const eventTarget = new EventTarget();

test/wpt/status/dom/events.json

@@ -11,6 +11,13 @@
   "Event-dispatch-listener-order.window.js": {
     "skip": "document is not defined"
   },
+  "Event-isTrusted.any.js": {
+    "fail": {
+      "expected": [
+        "Event-isTrusted"
+      ]
+    }
+  },
   "EventListener-addEventListener.sub.window.js": {
     "skip": "document is not defined"
   },