firebase-tools: Database emulator does not support .indexOn

[REQUIRED] Environment info

firebase-tools: 3.3.0

Platform: Windows

[REQUIRED] Test case

I used the official docs example for orderByChild https://firebase.google.com/docs/database/security/indexing-data?authuser=0.

// client code
      db.ref('dinosaurs').set({
        "lambeosaurus": {
          "height": 2.1,
          "length": 12.5,
          "weight": 5000
        },
        "stegosaurus": {
          "height": 4,
          "length": 9,
          "weight": 2500
        }
      }).then(()=>{
        db.ref('dinosaurs').orderByChild('height').startAt(3).once('value').then(s => console.log(s.val()));
      })

// security rules
{
  "rules": {
    ".read": true,
    ".write": true,
    "dinosaurs": {
      ".indexOn": [
        "height",
        "length"
      ]
    }
  }
}

[REQUIRED] Steps to reproduce

Running the code above against a real database shows stegosaurus: {height: 4, length: 9, weight: 2500}.

Running it against the emulator shows both stegosaurus: {height: 4, length: 9, weight: 2500} and a warning:

logger.ts:86 [2020-02-18T17:08:23.851Z]  @firebase/database: FIREBASE WARNING: Using an unspecified index. Your data will be downloaded and filtered on the client. Consider adding ".indexOn": "height" at /dinosaurs to your security rules for better performance. 

[REQUIRED] Expected behavior

I expected .indexOn to be supported, or perhaps for the emulator to mention it was not using the index, or for this limitation to be listed somewhere. Maybe it’s already documented but I couldn’t find it in the official docs or the issue tracker.

[REQUIRED] Actual behavior

The emulator silently ignores the index and the client warns that there is no index.