chore: revert revert of locker flag (#5230)

packages/@lwc/engine-core/src/framework/invoker.ts

@@ -11,7 +11,7 @@ import { addErrorComponentStack } from '../shared/error';
 import { evaluateTemplate, setVMBeingRendered, getVMBeingRendered } from './template';
 import { runWithBoundaryProtection } from './vm';
 import { logOperationStart, logOperationEnd, OperationId } from './profiler';
-import { LightningElement } from './base-lightning-element';
+import type { LightningElement } from './base-lightning-element';
 import type { Template } from './template';
 import type { VM } from './vm';
 import type { LightningElementConstructor } from './base-lightning-element';
@@ -58,9 +58,12 @@ export function invokeComponentConstructor(vm: VM, Ctor: LightningElementConstru
         // the "instanceof" operator would not work here since Locker Service provides its own
         // implementation of LightningElement, so we indirectly check if the base constructor is
         // invoked by accessing the component on the vm.
-        const isInvalidConstructor = lwcRuntimeFlags.LEGACY_LOCKER_ENABLED
-            ? vmBeingConstructed.component !== result
-            : !(result instanceof LightningElement);
+
+        // TODO [W-17769475]: Restore this fix when we can reliably detect Locker enabled
+        // const isInvalidConstructor = lwcRuntimeFlags.LEGACY_LOCKER_ENABLED
+        //     ? vmBeingConstructed.component !== result
+        //     : !(result instanceof LightningElement);
+        const isInvalidConstructor = vmBeingConstructed.component !== result;
 
         if (isInvalidConstructor) {
             throw new TypeError(

packages/@lwc/integration-karma/test/component/LightningElement/index.spec.js

@@ -81,7 +81,8 @@ it("[W-6981076] shouldn't throw when a component with an invalid child in unmoun
     expect(() => document.body.removeChild(elm)).not.toThrow();
 });
 
-it('should fail when the constructor returns something other than an instance of itself', () => {
+// TODO [W-17769475]: Restore this test when we can reliably detect Locker enabled
+xit('should fail when the constructor returns something other than an instance of itself', () => {
     expect(() => {
         createElement('x-returning-bad', { is: ReturningBad });
     }).toThrowError(