Fixed tab scrolling

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

diff --git a/packages/flutter/lib/src/material/tabs.dart b/packages/flutter/lib/src/material/tabs.dart
index e560d10..794c266 100644
--- a/packages/flutter/lib/src/material/tabs.dart
+++ b/packages/flutter/lib/src/material/tabs.dart
@@ -704,7 +704,8 @@
   void _updateScrollBehavior() {
     scrollTo(scrollBehavior.updateExtents(
       containerExtent: config.scrollDirection == Axis.vertical ? _viewportSize.height : _viewportSize.width,
-      contentExtent: _tabWidths.reduce((double sum, double width) => sum + width)
+      contentExtent: _tabWidths.reduce((double sum, double width) => sum + width),
+      scrollOffset: scrollOffset
     ));
   }
 
@@ -780,7 +781,7 @@
     );
 
     if (config.isScrollable) {
-      child: new Viewport(
+      return new Viewport(
         scrollDirection: Axis.horizontal,
         paintOffset: scrollOffsetToPixelDelta(scrollOffset),
         onPaintOffsetUpdateNeeded: _handlePaintOffsetUpdateNeeded,

packages/flutter/test/widget/tabs_test.dart

diff --git a/packages/flutter/test/widget/tabs_test.dart b/packages/flutter/test/widget/tabs_test.dart
index ed1d45d..a8ded32 100644
--- a/packages/flutter/test/widget/tabs_test.dart
+++ b/packages/flutter/test/widget/tabs_test.dart
@@ -7,12 +7,13 @@
 import 'package:flutter/widgets.dart';
 import 'package:test/test.dart';
 
-Widget buildFrame({ List<String> tabs, String value, bool isScrollable: false }) {
+Widget buildFrame({ List<String> tabs, String value, bool isScrollable: false, Key tabBarKey }) {
   return new Material(
     child: new TabBarSelection<String>(
       value: value,
       values: tabs,
       child: new TabBar<String>(
+        key: tabBarKey,
         labels: new Map<String, TabLabel>.fromIterable(tabs, value: (String tab) => new TabLabel(text: tab)),
         isScrollable: isScrollable
       )
@@ -98,4 +99,48 @@
       expect(selection.value, equals('A'));
     });
   });
+
+  test('Scrollable TabBar tap centers selected tab', () {
+    testWidgets((WidgetTester tester) {
+      List<String> tabs = <String>['AAAAAA', 'BBBBBB', 'CCCCCC', 'DDDDDD', 'EEEEEE', 'FFFFFF', 'GGGGGG', 'HHHHHH', 'IIIIII', 'JJJJJJ', 'KKKKKK', 'LLLLLL'];
+      Key tabBarKey = new Key('TabBar');
+      tester.pumpWidget(buildFrame(tabs: tabs, value: 'AAAAAA', isScrollable: true, tabBarKey: tabBarKey));
+      TabBarSelectionState<String> selection = TabBarSelection.of(tester.findText('AAAAAA'));
+      expect(selection, isNotNull);
+      expect(selection.value, equals('AAAAAA'));
+
+      expect(tester.getSize(tester.findElementByKey(tabBarKey)).width, equals(800.0));
+      // The center of the FFFFFF item is to the right of the TabBar's center
+      expect(tester.getCenter(tester.findText('FFFFFF')).x, greaterThan(401.0));
+
+      tester.tap(tester.findText('FFFFFF'));
+      tester.pump();
+      tester.pump(const Duration(seconds: 1)); // finish the scroll animation
+      expect(selection.value, equals('FFFFFF'));
+      // The center of the FFFFFF item is now at the TabBar's center
+      expect(tester.getCenter(tester.findText('FFFFFF')).x, closeTo(400.0, 1.0));
+    });
+  });
+
+
+  test('TabBar can be scrolled independent of the selection', () {
+    testWidgets((WidgetTester tester) {
+      List<String> tabs = <String>['AAAAAA', 'BBBBBB', 'CCCCCC', 'DDDDDD', 'EEEEEE', 'FFFFFF', 'GGGGGG', 'HHHHHH', 'IIIIII', 'JJJJJJ', 'KKKKKK', 'LLLLLL'];
+      Key tabBarKey = new Key('TabBar');
+      tester.pumpWidget(buildFrame(tabs: tabs, value: 'AAAAAA', isScrollable: true, tabBarKey: tabBarKey));
+      TabBarSelectionState<String> selection = TabBarSelection.of(tester.findText('AAAAAA'));
+      expect(selection, isNotNull);
+      expect(selection.value, equals('AAAAAA'));
+
+      // Fling-scroll the TabBar to the left
+      expect(tester.getCenter(tester.findText('HHHHHH')).x, lessThan(700.0));
+      tester.fling(tester.findElementByKey(tabBarKey), const Offset(-20.0, 0.0), 1000.0);
+      tester.pump();
+      tester.pump(const Duration(seconds: 1)); // finish the scroll animation
+      expect(tester.getCenter(tester.findText('HHHHHH')).x, lessThan(500.0));
+
+      // Scrolling the TabBar doesn't change the selection
+      expect(selection.value, equals('AAAAAA'));
+    });
+  });
 }