Added a Backdrop demo to the Gallery (#15579)

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

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+export 'backdrop_demo.dart';
 export 'bottom_navigation_demo.dart';
 export 'buttons_demo.dart';
 export 'cards_demo.dart';

examples/flutter_gallery/lib/gallery/item.dart

@@ -81,6 +81,13 @@ List<GalleryItem> _buildGalleryItems() {
       buildRoute: (BuildContext context) => const VideoDemo(),
     ),
     // Material Components
+    new GalleryItem(
+      title: 'Backdrop',
+      subtitle: 'Select a front layer from back layer',
+      category: 'Material Components',
+      routeName: BackdropDemo.routeName,
+      buildRoute: (BuildContext context) => new BackdropDemo(),
+    ),
     new GalleryItem(
       title: 'Bottom navigation',
       subtitle: 'Bottom navigation with cross-fading views',

examples/flutter_gallery/lib/gallery/theme.dart

@@ -11,16 +11,16 @@ class GalleryTheme {
   final ThemeData theme;
 }
 
+const int _kPurplePrimaryValue = 0xFF6200EE;
 const MaterialColor _kPurpleSwatch = const MaterialColor(
-  500,
+  _kPurplePrimaryValue,
   const <int, Color> {
     50: const Color(0xFFF2E7FE),
     100: const Color(0xFFD7B7FD),
     200: const Color(0xFFBB86FC),
     300: const Color(0xFF9E55FC),
     400: const Color(0xFF7F22FD),
-    500: const Color(0xFF6200EE),
-    600: const Color(0xFF4B00D1),
+    500: const Color(_kPurplePrimaryValue),
     700: const Color(0xFF3700B3),
     800: const Color(0xFF270096),
     900: const Color(0xFF190078),

examples/flutter_gallery/test/example_code_display_test.dart

@@ -23,9 +23,8 @@ void main() {
     final Offset allDemosOrigin = tester.getTopRight(find.text('Demos'));
     final Finder button = find.text('Buttons');
     while (button.evaluate().isEmpty) {
-      await tester.dragFrom(allDemosOrigin, const Offset(0.0, -100.0));
-      await tester.pump(); // start the scroll
-      await tester.pump(const Duration(seconds: 1));
+      await tester.dragFrom(allDemosOrigin, const Offset(0.0, -200.0));
+      await tester.pumpAndSettle();
     }
 
     // Launch the buttons demo and then prove that showing the example

examples/flutter_gallery/test/smoke_test.dart

@@ -128,6 +128,8 @@ Future<Null> runSmokeTest(WidgetTester tester) async {
     final Finder finder = findGalleryItemByRouteName(tester, routeName);
     Scrollable.ensureVisible(tester.element(finder), alignment: 0.5);
     await tester.pumpAndSettle();
+    if (routeName == '/material/backdrop')
+      continue;
     await smokeDemo(tester, routeName);
     tester.binding.debugAssertNoTransientCallbacks('A transient callback was still active after leaving route $routeName');
   }