Allow disabling the performance overlay in main.dart, fixes #5580 (#5583)

examples/flutter_gallery/lib/gallery/app.dart

@@ -26,9 +26,14 @@ final ThemeData _kGalleryDarkTheme = new ThemeData(
 );
 
 class GalleryApp extends StatefulWidget {
-  GalleryApp({this.updateUrlFetcher, Key key}) : super(key: key);
+  GalleryApp({
+    this.updateUrlFetcher,
+    this.enablePerformanceOverlay: true,
+    Key key}
+  ) : super(key: key);
 
   final UpdateUrlFetcher updateUrlFetcher;
+  final bool enablePerformanceOverlay;
 
   @override
   GalleryAppState createState() => new GalleryAppState();
@@ -56,11 +61,11 @@ class GalleryAppState extends State<GalleryApp> {
         });
       },
       showPerformanceOverlay: _showPerformanceOverlay,
-      onShowPerformanceOverlayChanged: (bool value) {
+      onShowPerformanceOverlayChanged: config.enablePerformanceOverlay ? (bool value) {
         setState(() {
           _showPerformanceOverlay = value;
         });
-      },
+      } : null,
       timeDilation: timeDilation,
       onTimeDilationChanged: (double value) {
         setState(() {

examples/flutter_gallery/lib/gallery/drawer.dart

@@ -162,7 +162,7 @@ class GalleryDrawer extends StatelessWidget {
               ]
             )
           ),
-          new DrawerItem(
+          onShowPerformanceOverlayChanged == null ? new Container(height: 0.0) : new DrawerItem(
             icon: new Icon(Icons.assessment),
             onPressed: () { onShowPerformanceOverlayChanged(!showPerformanceOverlay); },
             selected: showPerformanceOverlay,

examples/flutter_gallery/lib/gallery/home.dart

@@ -80,7 +80,6 @@ class GalleryHome extends StatefulWidget {
   }) : super(key: key) {
     assert(onThemeChanged != null);
     assert(onTimeDilationChanged != null);
-    assert(onShowPerformanceOverlayChanged != null);
   }
 
   final bool useLightTheme;