collection method
Gets a CollectionReference for the specified Firestore path.
Implementation
CollectionReference collection(String collectionPath) {
assert(collectionPath.isNotEmpty,
'a collectionPath path must be a non-empty string');
assert(!collectionPath.contains('//'),
'a collection path must not contain "//"');
assert(isValidCollectionPath(collectionPath),
'a collection path must point to a valid collection.');
return CollectionReference._(this, _delegate.collection(collectionPath));
}