Throw an exception in `Image.toByteData` rather than hang forever. (#15152)

lib/web_ui/lib/src/engine/html_image_codec.dart

@@ -136,6 +136,11 @@ class HtmlImage implements ui.Image {
     }
   }
 
+  // TODO(het): Support this for asset images and images generated from
+  // `Picture`s.
   /// Returns an error message on failure, null on success.
-  String _toByteData(int format, Callback<Uint8List> callback) => null;
+  String _toByteData(int format, Callback<Uint8List> callback) {
+    callback(null);
+    return 'Image.toByteData is not supported in Flutter for Web';
+  }
 }