Tabs occasionally tried to setState after dispose (#5784)

Fixes https://github.com/flutter/flutter/issues/5776

dev/benchmarks/microbenchmarks/README.md

@@ -3,10 +3,10 @@
 To run these benchmarks on a device:
 
 ```
-flutter run --release -t lib/gestures/velocity_tracker_data.dart
-flutter run --release -t lib/stocks/animation_bench.dart
-flutter run --release -t lib/stocks/build_bench.dart
-flutter run --release -t lib/stocks/layout_bench.dart
+flutter run --release lib/gestures/velocity_tracker_data.dart
+flutter run --release lib/stocks/animation_bench.dart
+flutter run --release lib/stocks/build_bench.dart
+flutter run --release lib/stocks/layout_bench.dart
 ```
 
 The results should be in the device logs.

dev/benchmarks/microbenchmarks/lib/stocks/layout_bench.dart

@@ -11,7 +11,6 @@ const Duration kBenchmarkTime = const Duration(seconds: 15);
 
 
 Future<Null> main() async {
-  assert(false); // don't run this in checked mode! Use --release.
   stock_data.StockDataFetcher.actuallyFetchData = false;
 
   final Stopwatch watch = new Stopwatch();

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

@@ -977,10 +977,12 @@ class _TabBarState<T> extends ScrollableState<TabBar<T>> implements TabBarSelect
     _indicatorRect = _selection != null ? _tabIndicatorRect(_selection.index) : Rect.zero;
     _updateScrollBehavior();
     SchedulerBinding.instance.addPostFrameCallback((Duration timeStamp) {
-      setState(() {
-        // the changes were made at layout time
-        // TODO(ianh): remove this setState: https://github.com/flutter/flutter/issues/5749
-      });
+      if (mounted) {
+        setState(() {
+          // the changes were made at layout time
+          // TODO(ianh): remove this setState: https://github.com/flutter/flutter/issues/5749
+        });
+      }
     });
   }