Override first, last and single on RealmResults

Avoid instantiating a _RealmResultsIterator for first and
single calls, and avoid iterating the entire collection for last.

lib/src/results.dart

@@ -16,7 +16,6 @@
 //
 ////////////////////////////////////////////////////////////////////////////////
 import 'dart:async';
-import 'dart:collection' as collection;
 import 'dart:ffi';
 
 import 'collections.dart';
@@ -28,7 +27,7 @@ import 'realm_object.dart';
 /// added to or deleted from the Realm that match the underlying query.
 ///
 /// {@category Realm}
-class RealmResults<T extends Object?> extends collection.IterableBase<T> with RealmEntity implements Finalizable {
+class RealmResults<T extends Object?> extends Iterable<T> with RealmEntity implements Finalizable {
  final RealmObjectMetadata? _metadata;
  final RealmResultsHandle _handle;
 
@@ -56,11 +55,9 @@ class RealmResults<T extends Object?> extends collection.IterableBase<T> with Re
  }
  }
 
- /// `true` if the `Results` collection is empty.
  @override
  bool get isEmpty => length == 0;
 
- /// Returns a new `Iterator` that allows iterating the elements in this `RealmResults`.
  @override
  Iterator<T> get iterator {
  var results = this;
@@ -71,10 +68,23 @@ class RealmResults<T extends Object?> extends collection.IterableBase<T> with Re
  return _RealmResultsIterator(results);
  }
 
- /// The number of values in this `Results` collection.
  @override
  int get length => realmCore.getResultsCount(this);
 
+ @override
+ T get first => this[0];
+
+ @override
+ T get last => this[length - 1];
+
+ @override
+ T get single {
+ if (length != 1) {
+ throw StateError('Expected exactly one element but was $length');
+ }
+ return this[0];
+ }
+
  /// Creates a frozen snapshot of this query.
  RealmResults<T> freeze() {
  if (isFrozen) {