Add DOM TouchEvent modifiers accessors

lib/web_ui/lib/src/engine/dom.dart

@@ -1142,6 +1142,10 @@ extension DomWheelEventExtension on DomWheelEvent {
 class DomTouchEvent extends DomUIEvent {}
 
 extension DomTouchEventExtension on DomTouchEvent {
+  external bool get altKey;
+  external bool get ctrlKey;
+  external bool get metaKey;
+  external bool get shiftKey;
   List<DomTouch>? get changedTouches => js_util
       .getProperty<List<Object?>?>(this, 'changedTouches')
       ?.cast<DomTouch>();

lib/web_ui/test/engine/pointer_binding_test.dart

@@ -234,6 +234,19 @@ void testMain() {
     expect(events[1].buttons, equals(0));
     expect(events[1].client.x, equals(0));
     expect(events[1].client.y, equals(0));
+
+    context.pressAllModifiers();
+    event = expectCorrectType(context.primaryDown(clientX: 100, clientY: 101));
+    expect(event.getModifierState('Alt'), true);
+    expect(event.getModifierState('Control'), true);
+    expect(event.getModifierState('Meta'), true);
+    expect(event.getModifierState('Shift'), true);
+    context.unpressAllModifiers();
+    event = expectCorrectType(context.primaryDown(clientX: 100, clientY: 101));
+    expect(event.getModifierState('Alt'), false);
+    expect(event.getModifierState('Control'), false);
+    expect(event.getModifierState('Meta'), false);
+    expect(event.getModifierState('Shift'), false);
   });
 
   test('_TouchEventContext generates expected events', () {
@@ -330,6 +343,19 @@ void testMain() {
     expect(events[0].changedTouches![1].identifier, equals(105));
     expect(events[0].changedTouches![1].client.x, equals(322));
     expect(events[0].changedTouches![1].client.y, equals(323));
+
+    context.pressAllModifiers();
+    event = expectCorrectType(context.primaryDown(clientX: 100, clientY: 101));
+    expect(event.altKey, true);
+    expect(event.ctrlKey, true);
+    expect(event.metaKey, true);
+    expect(event.shiftKey, true);
+    context.unpressAllModifiers();
+    event = expectCorrectType(context.primaryDown(clientX: 100, clientY: 101));
+    expect(event.altKey, false);
+    expect(event.ctrlKey, false);
+    expect(event.metaKey, false);
+    expect(event.shiftKey, false);
   });
 
   test('_MouseEventContext generates expected events', () {
@@ -419,6 +445,19 @@ void testMain() {
     expect(event.buttons, equals(0));
     expect(event.client.x, equals(400));
     expect(event.client.y, equals(401));
+
+    context.pressAllModifiers();
+    event = expectCorrectType(context.primaryDown(clientX: 100, clientY: 101));
+    expect(event.getModifierState('Alt'), true);
+    expect(event.getModifierState('Control'), true);
+    expect(event.getModifierState('Meta'), true);
+    expect(event.getModifierState('Shift'), true);
+    context.unpressAllModifiers();
+    event = expectCorrectType(context.primaryDown(clientX: 100, clientY: 101));
+    expect(event.getModifierState('Alt'), false);
+    expect(event.getModifierState('Control'), false);
+    expect(event.getModifierState('Meta'), false);
+    expect(event.getModifierState('Shift'), false);
   });
 
   // ALL ADAPTERS
@@ -2420,6 +2459,15 @@ abstract class _BasicEventContext implements PointerSupportDetector {
 
   bool get isSupported;
 
+  // Accepted modifier keys are 'Alt', 'Control', 'Meta' and 'Shift'.
+  // https://www.w3.org/TR/uievents-key/#keys-modifier defines more modifiers,
+  // but only the four main modifiers could be set from MouseEvent, PointerEvent
+  // and TouchEvent constructors.
+  bool altPressed = false;
+  bool ctrlPressed = false;
+  bool metaPressed = false;
+  bool shitPressed = false;
+
   // Generate an event that is:
   //
   //  * For mouse, a left click
@@ -2437,6 +2485,20 @@ abstract class _BasicEventContext implements PointerSupportDetector {
   //  * For mouse, release LMB
   //  * For touch, a touch up
   DomEvent primaryUp({double clientX, double clientY});
+
+  void pressAllModifiers() {
+    altPressed = true;
+    ctrlPressed = true;
+    metaPressed = true;
+    shitPressed = true;
+  }
+
+  void unpressAllModifiers() {
+    altPressed = false;
+    ctrlPressed = false;
+    metaPressed = false;
+    shitPressed = false;
+  }
 }
 
 mixin _ButtonedEventMixin on _BasicEventContext {
@@ -2622,6 +2684,10 @@ class _TouchEventContext extends _BasicEventContext
               ),
             )
             .toList(),
+        'altKey': altPressed,
+        'ctrlKey': ctrlPressed,
+        'metaKey': metaPressed,
+        'shiftKey': shitPressed,
       },
     );
   }
@@ -2757,6 +2823,10 @@ class _MouseEventContext extends _BasicEventContext
         'button': button,
         'clientX': clientX,
         'clientY': clientY,
+        'altKey': altPressed,
+        'ctrlKey': ctrlPressed,
+        'metaKey': metaPressed,
+        'shiftKey': shitPressed,
       }
     ];
     return js_util.callConstructor<DomMouseEvent>(
@@ -2835,6 +2905,10 @@ class _PointerEventContext extends _BasicEventContext
       'clientX': clientX,
       'clientY': clientY,
       'pointerType': pointerType,
+      'altKey': altPressed,
+      'ctrlKey': ctrlPressed,
+      'metaKey': metaPressed,
+      'shiftKey': shitPressed,
     });
   }