Make DoubleLinkedQueue.firstEntry/lastEntry nullable again.

It was migrated to null-safety by throwing, but that's an unnecessary breaking change.

Change-Id: I92c21d7518bf7c291b333b33a04eb4b21b7cc210
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/134725
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Lasse R.H. Nielsen <lrn@google.com>

Author: lrhn

sdk/lib/collection/queue.dart

@@ -400,23 +400,27 @@ class DoubleLinkedQueue<E> extends Iterable<E> implements Queue<E> {
   /// The entry object of the first element in the queue.
   ///
   /// Each element of the queue has an associated [DoubleLinkedQueueEntry].
-  /// Returns the entry object corresponding to the first element of the queue.
+  ///
+  /// Returns the entry object corresponding to the first element of the queue,
+  /// or `null` if the queue is empty.
   ///
   /// The entry objects can also be accessed using [lastEntry],
-  /// and they can be iterated using [DoubleLinkedQueueEntry.nextEntry()] and
-  /// [DoubleLinkedQueueEntry.previousEntry()].
+  /// and they can be iterated using [DoubleLinkedQueueEntry.nextEntry] and
+  /// [DoubleLinkedQueueEntry.previousEntry].
   DoubleLinkedQueueEntry<E> firstEntry() {
     return _sentinel.nextEntry();
   }
 
   /// The entry object of the last element in the queue.
   ///
   /// Each element of the queue has an associated [DoubleLinkedQueueEntry].
-  /// Returns the entry object corresponding to the last element of the queue.
+  ///
+  /// Returns the entry object corresponding to the last element of the queue,
+  /// or `null` if the queue is empty.
   ///
   /// The entry objects can also be accessed using [firstEntry],
-  /// and they can be iterated using [DoubleLinkedQueueEntry.nextEntry()] and
-  /// [DoubleLinkedQueueEntry.previousEntry()].
+  /// and they can be iterated using [DoubleLinkedQueueEntry.nextEntry] and
+  /// [DoubleLinkedQueueEntry.previousEntry].
   DoubleLinkedQueueEntry<E> lastEntry() {
     return _sentinel.previousEntry();
   }

sdk_nnbd/lib/collection/queue.dart

@@ -412,25 +412,29 @@ class DoubleLinkedQueue<E> extends Iterable<E> implements Queue<E> {
   /// The entry object of the first element in the queue.
   ///
   /// Each element of the queue has an associated [DoubleLinkedQueueEntry].
-  /// Returns the entry object corresponding to the first element of the queue.
+  ///
+  /// Returns the entry object corresponding to the first element of the queue,
+  /// or `null` if the queue is empty.
   ///
   /// The entry objects can also be accessed using [lastEntry],
-  /// and they can be iterated using [DoubleLinkedQueueEntry.nextEntry()] and
-  /// [DoubleLinkedQueueEntry.previousEntry()].
-  DoubleLinkedQueueEntry<E> firstEntry() {
-    return _sentinel.nextEntry()!;
+  /// and they can be iterated using [DoubleLinkedQueueEntry.nextEntry] and
+  /// [DoubleLinkedQueueEntry.previousEntry].
+  DoubleLinkedQueueEntry<E>? firstEntry() {
+    return _sentinel.nextEntry();
   }
 
   /// The entry object of the last element in the queue.
   ///
   /// Each element of the queue has an associated [DoubleLinkedQueueEntry].
-  /// Returns the entry object corresponding to the last element of the queue.
+  ///
+  /// Returns the entry object corresponding to the last element of the queue,
+  /// or `null` if the queue is empty.
   ///
   /// The entry objects can also be accessed using [firstEntry],
-  /// and they can be iterated using [DoubleLinkedQueueEntry.nextEntry()] and
-  /// [DoubleLinkedQueueEntry.previousEntry()].
-  DoubleLinkedQueueEntry<E> lastEntry() {
-    return _sentinel.previousEntry()!;
+  /// and they can be iterated using [DoubleLinkedQueueEntry.nextEntry] and
+  /// [DoubleLinkedQueueEntry.previousEntry].
+  DoubleLinkedQueueEntry<E>? lastEntry() {
+    return _sentinel.previousEntry();
   }
 
   bool get isEmpty {