[web] Fix Altgr keyboard crash (#21781)

* Fix AltGr modifier crash
* update integration test

e2etests/web/regular_integration_tests/test_driver/text_editing_integration.dart

@@ -179,6 +179,21 @@ void main() {
       'cancelable': true,
     });
 
+    // Press and release AltGr key.
+    // Regression test for https://github.com/flutter/flutter/issues/58979.
+    dispatchKeyboardEvent(input, 'keydown', <String, dynamic>{
+      'key': 'AltGraph',
+      'code': 'AltRight',
+      'bubbles': true,
+      'cancelable': true,
+    });
+    dispatchKeyboardEvent(input, 'keyup', <String, dynamic>{
+      'key': 'AltGraph',
+      'code': 'AltRight',
+      'bubbles': true,
+      'cancelable': true,
+    });
+
     // Press Tab. The focus should move to the next TextFormField.
     dispatchKeyboardEvent(input, 'keydown', <String, dynamic>{
       'key': 'Tab',

lib/web_ui/lib/src/engine/keyboard.dart

@@ -177,7 +177,7 @@ int _getMetaState(html.KeyboardEvent event) {
   if (event.getModifierState('Shift')) {
     metaState |= _modifierShift;
   }
-  if (event.getModifierState('Alt')) {
+  if (event.getModifierState('Alt') || event.getModifierState('AltGraph')) {
     metaState |= _modifierAlt;
   }
   if (event.getModifierState('Control')) {