add missing trailing commas (in examples/) (#40701)

examples/flutter_gallery/lib/demo/colors_demo.dart

@@ -117,7 +117,7 @@ class PaletteTabView extends StatelessWidget {
               );
             }),
         ],
-      )
+      ),
     );
   }
 }

examples/flutter_gallery/lib/demo/material/backdrop_demo.dart

@@ -140,7 +140,7 @@ class CategoryView extends StatelessWidget {
             ],
           );
         }).toList(),
-      )
+      ),
     );
   }
 }

examples/flutter_gallery/lib/demo/material/banner_demo.dart

@@ -57,7 +57,7 @@ class _BannerDemoState extends State<BannerDemo> {
             setState(() {
               _displayBanner = false;
             });
-          }
+          },
         ),
         if (_showMultipleActions)
           FlatButton(
@@ -66,7 +66,7 @@ class _BannerDemoState extends State<BannerDemo> {
               setState(() {
                 _displayBanner = false;
               });
-            }
+            },
           ),
       ],
     );

examples/flutter_gallery/lib/demo/material/snack_bar_demo.dart

@@ -88,7 +88,7 @@ class _SnackBarDemoState extends State<SnackBarDemo> {
         tooltip: 'Create',
         onPressed: () {
           print('Floating Action Button was pressed');
-        }
+        },
       ),
     );
   }

examples/layers/raw/canvas.dart

@@ -46,7 +46,7 @@ ui.Picture paint(ui.Rect paintBounds) {
   // Clips the current transform
   canvas.clipRect(
     ui.Rect.fromLTRB(0, radius + 50, logicalSize.width, logicalSize.height),
-    clipOp: ui.ClipOp.difference
+    clipOp: ui.ClipOp.difference,
   );
 
   // Shifts the coordinate space of and rotates the current transform