Update BottomNavigationBar.didUpdateWidget() (#20890)

BottomNavigationBar's state needs to update _backgroundColor when its configuration changes.

packages/flutter/lib/src/material/bottom_navigation_bar.dart

diff --git a/packages/flutter/lib/src/material/bottom_navigation_bar.dart b/packages/flutter/lib/src/material/bottom_navigation_bar.dart
index 1885b9a..7601e95 100644
--- a/packages/flutter/lib/src/material/bottom_navigation_bar.dart
+++ b/packages/flutter/lib/src/material/bottom_navigation_bar.dart
@@ -415,6 +415,9 @@
       _controllers[oldWidget.currentIndex].reverse();
       _controllers[widget.currentIndex].forward();
     }
+
+    if (_backgroundColor != widget.items[widget.currentIndex].backgroundColor)
+      _backgroundColor = widget.items[widget.currentIndex].backgroundColor;
   }
 
   List<Widget> _createTiles() {

packages/flutter/test/material/bottom_navigation_bar_test.dart

diff --git a/packages/flutter/test/material/bottom_navigation_bar_test.dart b/packages/flutter/test/material/bottom_navigation_bar_test.dart
index 84bedee..0f4aca5 100644
--- a/packages/flutter/test/material/bottom_navigation_bar_test.dart
+++ b/packages/flutter/test/material/bottom_navigation_bar_test.dart
@@ -712,6 +712,64 @@
     expect(find.text('item 2'), findsNothing);
     expect(find.text('item 3'), findsNothing);
   });
+
+  testWidgets('BottomNavigationBar change backgroundColor test', (WidgetTester tester) async {
+    // Regression test for: https://github.com/flutter/flutter/issues/19653
+
+    Color _backgroundColor = Colors.red;
+
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: new StatefulBuilder(
+          builder: (BuildContext context, StateSetter setState) {
+            return new Scaffold(
+              body: new Center(
+                child: new RaisedButton(
+                  child: const Text('green'),
+                  onPressed: () {
+                    setState(() {
+                      _backgroundColor = Colors.green;
+                    });
+                  },
+                ),
+              ),
+              bottomNavigationBar: new BottomNavigationBar(
+                type: BottomNavigationBarType.shifting,
+                items: <BottomNavigationBarItem>[
+                  new BottomNavigationBarItem(
+                    title: const Text('Page 1'),
+                    backgroundColor: _backgroundColor,
+                    icon: const Icon(Icons.dashboard),
+                  ),
+                  new BottomNavigationBarItem(
+                    title: const Text('Page 2'),
+                    backgroundColor: _backgroundColor,
+                    icon: const Icon(Icons.menu),
+                  ),
+                ],
+              ),
+            );
+          },
+        ),
+      ),
+    );
+
+    final Finder backgroundMaterial = find.descendant(
+      of: find.byType(BottomNavigationBar),
+      matching: find.byWidgetPredicate((Widget w) {
+        if (w is Material)
+          return w.type == MaterialType.canvas;
+        return false;
+      }),
+    );
+
+    expect(_backgroundColor, Colors.red);
+    expect(tester.widget<Material>(backgroundMaterial).color, Colors.red);
+    await tester.tap(find.text('green'));
+    await tester.pumpAndSettle();
+    expect(_backgroundColor, Colors.green);
+    expect(tester.widget<Material>(backgroundMaterial).color, Colors.green);
+  });
 }
 
 Widget boilerplate({ Widget bottomNavigationBar, @required TextDirection textDirection }) {