Use typed get method in TestBed class

I’m submitting a…

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Performance issue
[x] 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
[ ] Other... Please describe:

Current behavior

The get static method of TestBed class does not return a typed object.

const service = TestBed.get(MyService); // service is typed as any

Expected behavior

The get static method should return a typed object.

const service = TestBed.get(MyService); // service should be typed as MyService

Minimal reproduction of the problem with instructions

Look examples.

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

This would improve type safety.

Environment

Angular version: 6.1.9

Others:

A possible implementation using new helper types from TypeScript would be:

    static get<T extends new (...args: any[]) => any>(token: T, notFoundValue?: T | InstanceType<T>): InstanceType<T>;

I’m not sure if the notFoundValue should be a constructor or an instance, so I typed it as both, but fell free to update it as is actually needed. I tested that signature and it actually returns a typed object as wanted.

[Goodwine]

Per discussion with @vikerman, the Angular team can’t accept such a breaking change at this point, so the approach we’ll now take is to create a new method TestBed.inject which will be type-safe. And deprecate TestBed.get.

[Goodwine]

Talking with the angular team (I’m not part of the angular team), I’ll actually send PRs in the following order:

1. Add @deprecate to current function signature and add same signature as Injector.get, however it would still return any.
   • This already happened on #13785 but now breaking changes aren’t as easy to make 😃
2. Add set of types that will eventually be used, even if they aren’t used yet.
3. Insert breaking changes that could land in a future major version, but definitely not Angular 8