[cloud_firestore_platform_interface] Pointer equality not normalising paths

[IchordeDionysos]

Describe the bug
Not sure did not test it yet. (Will write a test and open a PR if the problem is real).

The pointer implementation splits the path into components AND removes any empty element/removes leading/trailing slashes.

Which is basically what we want!
However, when checking for equality the implementation does not use any normalized form, but rather uses the user-provided path for equality checking, which leads to unexpected inequalities of document and collection references.

Look at the way DocumentReferences are created:

1. Android/iOS implementation: No normalization there. That's okay as I believe the normalization should not be done by the implementing library.
2. MethodChannelFirebaseFirestore: Not there either. I think it's okay.
3. MethodChannelCollectionReference: No normalization. It's okay.
4. Pointer: No normalization. This is problematic.

In the pointer, the low-level, internal representation of references there is no normalization of the path.

When checking for equality of two DocumentReferences/CollectionReferences the equality of the underlying Pointers is checked.

So, let's play it through:

1. DocumentReference.==: Path property of references is compared.
2. DocumentReference.path: Path is returned from Pointer.
3. Pointer.path: Path from Pointer is returned as provided by the user, see above steps.

To Reproduce
Steps to reproduce the behavior:

1. Import cloud_firestore
2. Use this source code:

final db = FirebaseFirestore.instance;
print(db.collection('test') == db.collection('/test'));

4. See that false is printed out.

Expected behavior
db.collection('test') == db.collection('/test') should evaluate to true.

Additional context
Discovered this bug while looking at the implementation of the Pointer class.

Will update this issue once I tested it out and validated it.

[IchordeDionysos]

Adding a test like this would fail:

test('Pointer equality with un-normalized paths', () {
  expect(Pointer('foo') == Pointer('/foo'), true);
  expect(Pointer('foo') == Pointer('/foo/bar'), false);
  expect(Pointer('foo') == Pointer('foo/'), true);
  expect(Pointer('foo') == Pointer('foo/bar/'), false);
});

[IchordeDionysos]

@Salakar I think this should block a release of rework of cloud_firestore :)

I will open a PR for it 😊

[Salakar]

cc @Ehesp

[Salakar]

@IchordeDionysos cool, good find! PR is welcome, let me know if you need any help with local setup, the contributing/local setup docs are still pending