Fix KeySet<T> (and LogicalKeySet, PhysicalKeySet) hashCode calculation (#38936)

This fixes the hashCode calculation for KeySet<T> so that it doesn't depend on the insertion order of the keys in the set.

The fix involves switching from Set<T> to HashSet<T> internally, so that the iteration order is stable around the hash values of the inserted keys, and not the insertion order. This matters when hashList is called in KeySet<T>.hashCode to build the hash value of the contents of the internal set.

Fixes #38919

packages/flutter/lib/src/widgets/shortcuts.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:collection';
+
 import 'package:collection/collection.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
@@ -39,7 +41,7 @@ class KeySet<T extends KeyboardKey> extends Diagnosticable {
     T key3,
     T key4,
   ])  : assert(key1 != null),
-        _keys = <T>{key1} {
+        _keys = HashSet<T>()..add(key1) {
     int count = 1;
     if (key2 != null) {
       _keys.add(key2);
@@ -74,11 +76,14 @@ class KeySet<T extends KeyboardKey> extends Diagnosticable {
       : assert(keys != null),
         assert(keys.isNotEmpty),
         assert(!keys.contains(null)),
-        _keys = keys;
+        _keys = HashSet<T>.from(keys);
 
   /// Returns an unmodifiable view of the [KeyboardKey]s in this [KeySet].
   Set<T> get keys => UnmodifiableSetView<T>(_keys);
-  final Set<T> _keys;
+  // This needs to be a hash set to be sure that the hashCode accessor returns
+  // consistent results. LinkedHashSet (the default Set implementation) depends
+  // upon insertion order, and HashSet does not.
+  final HashSet<T> _keys;
 
   @override
   bool operator ==(Object other) {

packages/flutter/test/widgets/shortcuts_test.dart

@@ -167,8 +167,26 @@ void main() {
       final LogicalKeySet set2 = LogicalKeySet(
         LogicalKeyboardKey.keyA,
         LogicalKeyboardKey.keyB,
+        LogicalKeyboardKey.keyC,
+        LogicalKeyboardKey.keyD,
+      );
+      final LogicalKeySet set3 = LogicalKeySet(
+        LogicalKeyboardKey.keyD,
+        LogicalKeyboardKey.keyC,
+        LogicalKeyboardKey.keyB,
+        LogicalKeyboardKey.keyA,
+      );
+      final LogicalKeySet set4 = LogicalKeySet.fromSet(<LogicalKeyboardKey>{
+        LogicalKeyboardKey.keyD,
+        LogicalKeyboardKey.keyC,
+        LogicalKeyboardKey.keyB,
+        LogicalKeyboardKey.keyA,}
       );
       final Map<LogicalKeySet, String> map = <LogicalKeySet, String>{set1: 'one'};
+      expect(set2 == set3, isTrue);
+      expect(set2 == set4, isTrue);
+      expect(set2.hashCode == set3.hashCode, isTrue);
+      expect(set2.hashCode == set4.hashCode, isTrue);
       expect(map.containsKey(set1), isTrue);
       expect(map.containsKey(LogicalKeySet(LogicalKeyboardKey.keyA)), isTrue);
       expect(
@@ -176,6 +194,8 @@ void main() {
           equals(LogicalKeySet.fromSet(<LogicalKeyboardKey>{
             LogicalKeyboardKey.keyA,
             LogicalKeyboardKey.keyB,
+            LogicalKeyboardKey.keyC,
+            LogicalKeyboardKey.keyD,
           })));
     });
     test('$KeySet diagnostics work.', () {