I created a PR (glimmerjs/glimmer-vm#1384) to add the needed feature flag on the Glimmer side: glimmerjs/glimmer-vm#1384

When I linked it into my local ember-source repo I did notice some tests were failing on the Ember side, so it probably requires more changes than simply bumping the version and enabling the feature. I might be wrong though, it was getting late 😅.

I released the flag @Windvis added as 0.84.1 so we can proceed with this (w.r.t. the remaining steps in glimmerjs/glimmer-vm#1383). Note that in Ember, we should default the feature to be off unless specifically turned on by the ember feature flag (the ember feature flag's value should in turns be used to set the glimmer flag @Windvis added)

Specifically:

1. Add flag to https://github.com/emberjs/ember.js/blob/master/packages/%40ember/canary-features/index.ts#L14-L18
2. In

   ember.js/packages/@ember/-internals/glimmer/lib/environment.ts

   Lines 21 to 88 in f89dd90

   	setGlobalContext({
   	scheduleRevalidate() {
   	_backburner.ensureInstance();
   	},
   	toBool,
   	toIterator,
   	getProp: _getProp,
   	setProp: _setProp,
   	getPath: get,
   	setPath: set,
   	scheduleDestroy(destroyable, destructor) {
   	schedule('actions', null, destructor, destroyable);
   	},
   	scheduleDestroyed(finalizeDestructor) {
   	schedule('destroy', null, finalizeDestructor);
   	},
   	warnIfStyleNotTrusted(value: unknown) {
   	warn(
   	constructStyleDeprecationMessage(String(value)),
   	(() => {
   	if (value === null || value === undefined || isHTMLSafe(value)) {
   	return true;
   	}
   	return false;
   	})(),
   	{ id: 'ember-htmlbars.style-xss-warning' }
   	);
   	},
   	assert(test: unknown, msg: string, options?: { id: string }) {
   	if (DEBUG) {
   	let id = options?.id;
   	let override = VM_ASSERTION_OVERRIDES.filter((o) => o.id === id)[0];
   	assert(override?.message ?? msg, test);
   	}
   	},
   	deprecate(msg: string, test: unknown, options: { id: string }) {
   	if (DEBUG) {
   	let { id } = options;
   	if (id === 'argument-less-helper-paren-less-invocation') {
   	throw new Error(
   	`A resolved helper cannot be passed as a named argument as the syntax is ` +
   	`ambiguously a pass-by-reference or invocation. Use the ` +
   	`\`{{helper 'foo-helper}}\` helper to pass by reference or explicitly ` +
   	`invoke the helper with parens: \`{{(fooHelper)}}\`.`
   	);
   	}
   	let override = VM_DEPRECATION_OVERRIDES.filter((o) => o.id === id)[0];
   	if (!override) throw new Error(`deprecation override for ${id} not found`);
   	// allow deprecations to be disabled in the VM_DEPRECATION_OVERRIDES array below
   	if (!override.disabled) {
   	deprecate(override.message ?? msg, Boolean(test), override);
   	}
   	}
   	},
   	});

   , set the value based on the flag
3. Add/update the relevant tests taking into account the value of the flag

This was superseded by #20052.