Update DefaultTabController to allow for zero tabs (#41625)

* Update DefaultTabController to allow for zero tabs

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

@@ -313,7 +313,7 @@ class DefaultTabController extends StatefulWidget {
     @required this.child,
   }) : assert(initialIndex != null),
        assert(length >= 0),
-       assert(initialIndex >= 0 && initialIndex < length),
+       assert(length == 0 || (initialIndex >= 0 && initialIndex < length)),
        super(key: key);
 
   /// The total number of tabs.

packages/flutter/test/material/tabs_test.dart

@@ -2350,4 +2350,77 @@ void main() {
     expect(find.text('Tab1'), findsOneWidget);
     expect(find.text('Tab2'), findsOneWidget);
   });
+
+  testWidgets('DefaultTabController should allow for a length of zero', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/20292.
+    List<String> tabTextContent = <String>[];
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: StatefulBuilder(
+          builder: (BuildContext context, StateSetter setState) {
+            return DefaultTabController(
+              length: tabTextContent.length,
+              child: Scaffold(
+                appBar: AppBar(
+                  title: const Text('Default TabBar Preview'),
+                  bottom: tabTextContent.isNotEmpty
+                    ? TabBar(
+                       isScrollable: true,
+                       tabs: tabTextContent.map((String textContent) => Tab(text: textContent)).toList(),
+                     )
+                    : null,
+                ),
+                body: tabTextContent.isNotEmpty
+                  ? TabBarView(
+                      children: tabTextContent.map((String textContent) => Tab(text: '$textContent\'s view')).toList()
+                    )
+                  : const Center(child: Text('No tabs')),
+                bottomNavigationBar: BottomAppBar(
+                  child: Row(
+                    mainAxisSize: MainAxisSize.max,
+                    mainAxisAlignment: MainAxisAlignment.spaceBetween,
+                    children: <Widget>[
+                      IconButton(
+                        key: const Key('Add tab'),
+                        icon: const Icon(Icons.add),
+                        onPressed: () {
+                          setState(() {
+                            tabTextContent = List<String>.from(tabTextContent)
+                              ..add('Tab ${tabTextContent.length + 1}');
+                          });
+                        },
+                      ),
+                      IconButton(
+                        key: const Key('Delete tab'),
+                        icon: const Icon(Icons.delete),
+                        onPressed: () {
+                          setState(() {
+                            tabTextContent = List<String>.from(tabTextContent)
+                              ..removeLast();
+                          });
+                        },
+                      ),
+                    ],
+                  ),
+                ),
+              ),
+            );
+          },
+        ),
+      ),
+    );
+
+    // Initializes with zero tabs properly
+    expect(find.text('No tabs'), findsOneWidget);
+    await tester.tap(find.byKey(const Key('Add tab')));
+    await tester.pumpAndSettle();
+    expect(find.text('Tab 1'), findsOneWidget);
+    expect(find.text('Tab 1\'s view'), findsOneWidget);
+
+    // Dynamically updates to zero tabs properly
+    await tester.tap(find.byKey(const Key('Delete tab')));
+    await tester.pumpAndSettle();
+    expect(find.text('No tabs'), findsOneWidget);
+  });
 }