[web] Set pointer-events: none for img-element-backed images (flutter#177418)

This stops the context menu from showing up when you right click on an
image that is backed by an <img> element.

Fixes internal issue

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Author: harryterkelsen

packages/flutter/lib/src/web.dart

@@ -42,6 +42,8 @@ extension type CSSStyleDeclaration._(JSObject _) implements JSObject {
   external String get height;
   external set width(String value);
   external String get width;
+  external set pointerEvents(String value);
+  external String get pointerEvents;
 }
 
 extension type CSSStyleSheet._(JSObject _) implements JSObject {

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

@@ -42,7 +42,8 @@ class ImgElementPlatformView extends StatelessWidget {
       // Set `width` and `height`, otherwise the engine will issue a warning.
       img.style
         ..width = '100%'
-        ..height = '100%';
+        ..height = '100%'
+        ..pointerEvents = 'none';
       return img;
     });
   }

packages/flutter/test/painting/_network_image_test_web.dart

@@ -471,6 +471,25 @@ void runTests() {
     expect(imgElement.style.height, '100%');
   });
 
+  testWidgets('Creates an <img> with pointer-events: none', (WidgetTester tester) async {
+    final TestImgElement testImg = TestImgElement();
+    testImg.src = _uniqueUrl(tester.testDescription);
+
+    final _TestImageStreamCompleter streamCompleter = _TestImageStreamCompleter();
+    final _TestImageProvider imageProvider = _TestImageProvider(streamCompleter: streamCompleter);
+
+    await tester.pumpWidget(Image(image: imageProvider));
+    streamCompleter.setData(
+      imageInfo: WebImageInfo(testImg.getMock() as web_shim.HTMLImageElement),
+    );
+    await tester.pumpAndSettle();
+    final FakePlatformView imgElementPlatformView = fakePlatformViewRegistry.views.single;
+    expect(imgElementPlatformView.htmlElement, isA<web.HTMLImageElement>());
+    final web.HTMLImageElement imgElement =
+        imgElementPlatformView.htmlElement as web.HTMLImageElement;
+    expect(imgElement.style.pointerEvents, 'none');
+  });
+
   group('RenderWebImage', () {
     testWidgets('BoxFit.contain centers and sizes the image correctly', (
       WidgetTester tester,