collection('...').document() should not save a document about fake_cloud_firestore

[suztomo]

Initial Attempt failed

Initial attempt that avoids modifying rootParent until updateData and setData failed.
suztomo@3ed76fe#diff-855d1299f8d5526ef0aacc2ddf24af06R28

This attempt fails to record subcollection whose parent is not saved.
cloud_firestore_mocks internally maintains documents as a tree using Map. When parent is not saved, getting collection reference creates a new map which does not have the subcolleciton.

This issue requires to save whether a document is stored in the instance or not. Maybe MockFirestoreInstance remembers all saved document location such as /users/t1gVCkBrgMU9LjFRnrtu1FFjlir1/persons/FkHWNY0ISMz9Zia0mF0q and MockDocumentReference and MockCollectionReference remembers MockFirestoreInstance.

[suztomo]

Note. In real Firestore, creating a non-top-level document does not create its parent document. For example, creating users/abc/friends/{documentId} does not create a document at users/abc.

    final firestore = Firestore.instance;
    await firestore.collection('dummy').document('abc')
    .collection('friends').document().setData(
      <String, dynamic>{
        'name': 'Tomo',
      }
    );

    final abcSaved = await firestore.collection('dummy').document('abc').get();
    if (abcSaved.exists) { // This is false.
      print('$abcSaved');
    }