Animation status callbacks should be re-entrant

We should mutate the status of the AnimationController before calling
the status callbacks so that we're prepared to be re-entered.

Fixes #1911

packages/flutter/lib/src/animation/animation_controller.dart

@@ -163,10 +163,11 @@ class AnimationController extends Animation<double>
 
   AnimationStatus _lastStatus = AnimationStatus.dismissed;
   void _checkStatusChanged() {
-    AnimationStatus currentStatus = status;
-    if (currentStatus != _lastStatus)
-      notifyStatusListeners(status);
-    _lastStatus = currentStatus;
+    AnimationStatus newStatus = status;
+    AnimationStatus oldStatus = _lastStatus;
+    _lastStatus = newStatus;
+    if (oldStatus != newStatus)
+      notifyStatusListeners(newStatus);
   }
 
   /// Drives the animation from its current value to target.

packages/flutter/test/animation/animation_controller_test.dart

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/animation.dart';
+import 'package:flutter/scheduler.dart';
+import 'package:flutter/widgets.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test("Can set value during status callback", () {
+    WidgetFlutterBinding.ensureInitialized();
+    AnimationController controller = new AnimationController(
+      duration: const Duration(milliseconds: 100)
+    );
+    bool didComplete = false;
+    bool didDismiss = false;
+    controller.addStatusListener((AnimationStatus status) {
+      if (status == AnimationStatus.completed) {
+        didComplete = true;
+        controller.value = 0.0;
+        controller.forward();
+      } else if (status == AnimationStatus.dismissed) {
+        didDismiss = true;
+        controller.value = 0.0;
+        controller.forward();
+      }
+    });
+
+    controller.forward();
+    expect(didComplete, isFalse);
+    expect(didDismiss, isFalse);
+    Scheduler.instance.handleBeginFrame(const Duration(seconds: 1));
+    expect(didComplete, isFalse);
+    expect(didDismiss, isFalse);
+    Scheduler.instance.handleBeginFrame(const Duration(seconds: 2));
+    expect(didComplete, isTrue);
+    expect(didDismiss, isTrue);
+  });
+}