Autotrack, take 2

[wycats]

This PR adds "auto-tracking" to tracked getters.

Previously, when defining a tracked computed property, you would write:

class {
  @tracked first;
  @tracked last;

  @tracked('first', 'last') get name() {
    return `${this.first} ${this.last}`;
  }
}

After this change, you can leave off the "dependent keys" and write:

class {
  @tracked first;
  @tracked last;

  @tracked get name() {
    return `${this.first} ${this.last}`;
  }
}

Rather than rely on knowing the full list of dependent properties up front, after this change Glimmer tracks the consumption of all @tracked properties inside the getter, and automatically creates a validator based on the consumed keys.

[mmun]

The code looks good to me. I've also written a benchmark here:

https://github.com/glimmerjs/glimmer.js/compare/autotrack...autotrack-benchmark?expand=1#diff-504d4e1545bcaa7fc620aec828f900b4

The results are great! There's basically no difference between @tracked with explicit dependencies and with auto-tracking.

[locks]

Yay 😍

[tomdale]

Implementation looks 👌 to me. @wycats, would you mind updating the comments and inline documentation to reflect the code changes? Happy to merge after that's done.

[tomdale]

Also, ideally, would you mind updating the body of the PR to explain to viewers at home what's going on here?

[wycats]

@tomdale done!

[sandstrom]

Out of curiosity, would this work too? 😄

class {
  first: '',
  last: '',

  @tracked get name() {
    return `${this.first} ${this.last}`;
  }
}

In other words, is it enough to tell the computed property that it's tracked, or must all dependent keys be marked as tracked too?

[rwjblue]

Anything not tracked is considered "constant", so in the above if you expect to update on changes from first or last you would need to track those also...