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[X] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question

Current behavior

I believe I've found a regression issue with QueryList, possibly caused by this commit: e544742#diff-a85dbe0991a7577ea24b49374e9ae90b.

I stumbled upon this in an isolated test, i.e., I'm just testing the class logic, not rendering the component. The component has a property decorated with @ViewChildren and has other logic that depends of the QueryList length. So for testing purposes, I "manually" instantiate a QueryList because, of course, it would be automatically populated when outside of Angular's purview. This is where the issue comes in:

Previously (Angular 5.1.3, in my case), queryListInstance.length used to have an initial value of 0 because it was a getter for the _result array length. Upon upgrading to Angular 5.2.3, it's now undefined. Now to actually have it populated, the use reset() is required:

const ql = new QueryList();
console.log(ql.length); // => undefined
ql.reset([]);
console.log(ql.length); // => 0

I've only been impacted by this in isolated tests. ViewChildren's logic, however, through it's use of QueryList's reset() makes the change imperceptible when the component is rendered.

Expected behavior

const ql = new QueryList();
console.log(ql.length); // => 0, without the need to call reset()

Minimal reproduction of the problem with instructions

The below examples show how QueryList behavior changed when using new QueryList(). Also showcased, QueryLists via @ViewChildren haven't changed in practice; however, in Angular 5.2.3, the length must be getting initialized via calling reset() behind the scenes.

With Angular 5.1.3: https://stackblitz.com/edit/angular-akgclx
With Angular 5.2.3: https://stackblitz.com/edit/angular-tcnwtn

What is the motivation / use case for changing the behavior?

Regression

Environment

Angular version: 5.2.3