Fix scollable tabs, add basic unit test

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

@@ -335,7 +335,7 @@ class Tab extends StatelessComponent {
     }
 
     Container centeredLabel = new Container(
-      child: new Center(child: labelContent),
+      child: new Center(child: labelContent, shrinkWrap: ShrinkWrap.both),
       constraints: new BoxConstraints(minWidth: _kMinTabWidth),
       padding: _kTabLabelPadding
     );

packages/unit/test/widget/tabs_test.dart

+import 'package:flutter/material.dart';
+import 'package:flutter/widgets.dart';
+import 'package:test/test.dart';
+
+import 'widget_tester.dart';
+
+int selectedIndex = 2;
+
+Widget buildFrame({ List<String> tabs, bool isScrollable: false }) {
+  return new TabBar(
+    labels: tabs.map((String tab) => new TabLabel(text: tab)).toList(),
+    selectedIndex: selectedIndex,
+    isScrollable: isScrollable,
+    onChanged: (tabIndex) {
+      selectedIndex = tabIndex;
+    }
+  );
+}
+
+void main() {
+  test('TabBar tap selects tab', () {
+    testWidgets((WidgetTester tester) {
+      List<String> tabs = <String>['A', 'B', 'C'];
+      selectedIndex = 2;
+
+      tester.pumpWidget(buildFrame(tabs: tabs, isScrollable: false));
+      expect(tester.findText('A'), isNotNull);
+      expect(tester.findText('B'), isNotNull);
+      expect(tester.findText('C'), isNotNull);
+      expect(selectedIndex, equals(2));
+
+      tester.pumpWidget(buildFrame(tabs: tabs, isScrollable: false));
+      tester.tap(tester.findText('B'));
+      tester.pump();
+      expect(selectedIndex, equals(1));
+
+      tester.pumpWidget(buildFrame(tabs: tabs, isScrollable: false));
+      tester.tap(tester.findText('C'));
+      tester.pump();
+      expect(selectedIndex, equals(2));
+
+      tester.pumpWidget(buildFrame(tabs: tabs, isScrollable: false));
+      tester.tap(tester.findText('A'));
+      tester.pump();
+      expect(selectedIndex, equals(0));
+    });
+  });
+
+  test('Scrollable TabBar tap selects tab', () {
+    testWidgets((WidgetTester tester) {
+      List<String> tabs = <String>['A', 'B', 'C'];
+      selectedIndex = 2;
+
+      tester.pumpWidget(buildFrame(tabs: tabs, isScrollable: true));
+      expect(tester.findText('A'), isNotNull);
+      expect(tester.findText('B'), isNotNull);
+      expect(tester.findText('C'), isNotNull);
+      expect(selectedIndex, equals(2));
+
+      tester.pumpWidget(buildFrame(tabs: tabs, isScrollable: true));
+      tester.tap(tester.findText('B'));
+      tester.pump();
+      expect(selectedIndex, equals(1));
+
+      tester.pumpWidget(buildFrame(tabs: tabs, isScrollable: true));
+      tester.tap(tester.findText('C'));
+      tester.pump();
+      expect(selectedIndex, equals(2));
+
+      tester.pumpWidget(buildFrame(tabs: tabs, isScrollable: true));
+      tester.tap(tester.findText('A'));
+      tester.pump();
+      expect(selectedIndex, equals(0));
+    });
+  });
+}