Make Tab take a widget as child (#12613)

* Make Tab take a widget as child

This way you can override the semantics information of a Tab (see test).

Fixes https://github.com/flutter/flutter/issues/12432

* more tests

* review comments

* review comment

* doc nits

* style fix

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

@@ -34,23 +34,35 @@ const double _kMaxTabWidth = 264.0;
 ///  * [TabController], which coordinates tab selection between a [TabBar] and a [TabBarView].
 ///  * <https://material.google.com/components/tabs.html>
 class Tab extends StatelessWidget {
-  /// Creates a material design [TabBar] tab. At least one of [text] and [icon]
-  /// must be non-null.
+  /// Creates a material design [TabBar] tab. At least one of [text], [icon],
+  /// and [child] must be non-null. The [text] and [child] arguments must not be
+  /// used at the same time.
   const Tab({
     Key key,
     this.text,
     this.icon,
-  }) : assert(text != null || icon != null),
+    this.child,
+  }) : assert(text != null || child != null || icon != null),
+       assert(!(text != null && null != child)), // TODO(goderbauer): https://github.com/dart-lang/sdk/issues/31140
        super(key: key);
 
   /// The text to display as the tab's label.
+  ///
+  /// Must not be used in combination with [child].
   final String text;
 
+  /// The widget to be used as the tab's label.
+  ///
+  /// Usually a [Text] widget, possibly wrapped in a [Semantics] widget.
+  ///
+  /// Must not be used in combination with [text].
+  final Widget child;
+
   /// An icon to display as the tab's label.
   final Widget icon;
 
   Widget _buildLabelText() {
-    return new Text(text, softWrap: false, overflow: TextOverflow.fade);
+    return child ?? new Text(text, softWrap: false, overflow: TextOverflow.fade);
   }
 
   @override

packages/flutter/test/material/tabs_test.dart

@@ -1218,4 +1218,75 @@ void main() {
 
     expect(controller.index, 1);
   });
+
+  testWidgets('can override semantics of tabs', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+
+    final List<Tab> tabs = new List<Tab>.generate(2, (int index) {
+      return new Tab(
+        child: new Semantics(
+          label: 'Semantics override $index',
+          child: new ExcludeSemantics(
+            child: new Text('TAB #$index'),
+          ),
+        ),
+      );
+    });
+
+    final TabController controller = new TabController(
+      vsync: const TestVSync(),
+      length: tabs.length,
+      initialIndex: 0,
+    );
+
+    await tester.pumpWidget(
+      boilerplate(
+        child: new Semantics(
+          container: true,
+          child: new TabBar(
+            isScrollable: true,
+            controller: controller,
+            tabs: tabs,
+          ),
+        ),
+      ),
+    );
+
+    final TestSemantics expectedSemantics = new TestSemantics.root(
+      children: <TestSemantics>[
+        new TestSemantics.rootChild(
+          id: 64,
+          rect: TestSemantics.fullScreen,
+          children: <TestSemantics>[
+            new TestSemantics(
+              id: 65,
+              actions: SemanticsAction.tap.index,
+              flags: SemanticsFlags.isSelected.index,
+              label: 'Semantics override 0\nTab 1 of 2',
+              rect: new Rect.fromLTRB(0.0, 0.0, 108.0, kTextTabBarHeight),
+              transform: new Matrix4.translationValues(0.0, 276.0, 0.0),
+            ),
+            new TestSemantics(
+              id: 68,
+              actions: SemanticsAction.tap.index,
+              label: 'Semantics override 1\nTab 2 of 2',
+              rect: new Rect.fromLTRB(0.0, 0.0, 108.0, kTextTabBarHeight),
+              transform: new Matrix4.translationValues(108.0, 276.0, 0.0),
+            ),
+          ],
+        ),
+      ],
+    );
+
+    expect(semantics, hasSemantics(expectedSemantics));
+
+    semantics.dispose();
+  });
+
+  test('illegal constructor combinations', () {
+    final Widget $null = null;
+    expect(() => new Tab(icon: $null), throwsAssertionError);
+    expect(() => new Tab(icon: new Container(), text: 'foo', child: new Container()), throwsAssertionError);
+    expect(() => new Tab(text: 'foo', child: new Container()), throwsAssertionError);
+  });
 }