Fixes TimePicker (#3583)

* Fixed TimePicker crash and added tests

* Fixed TimePicker crash and added tests

* fixed a Lint-O

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

diff --git a/packages/flutter/lib/src/material/time_picker.dart b/packages/flutter/lib/src/material/time_picker.dart
index 5fb1ec1..36a5748 100644
--- a/packages/flutter/lib/src/material/time_picker.dart
+++ b/packages/flutter/lib/src/material/time_picker.dart
@@ -106,9 +106,10 @@
 ///  * <https://www.google.com/design/spec/components/pickers.html#pickers-time-pickers>
 class TimePicker extends StatefulWidget {
   TimePicker({
+    Key key,
     this.selectedTime,
     this.onChanged
-  }) {
+  }) : super(key: key) {
     assert(selectedTime != null);
   }
 
@@ -417,10 +418,11 @@
 
   void _updateThetaForPan() {
     setState(() {
-      Offset offset = _position - _center;
+      final Offset offset = _position - _center;
+      final double angle = (math.atan2(offset.dx, offset.dy) - math.PI / 2.0) % _kTwoPi;
       _thetaTween
-        ..begin = (math.atan2(offset.dx, offset.dy) - math.PI / 2.0) % _kTwoPi
-        ..end = null;
+        ..begin = angle
+        ..end = angle; // The controller doesn't animate during the pan gesture.
     });
   }
 

packages/flutter/test/material/time_picker_test.dart

diff --git a/packages/flutter/test/material/time_picker_test.dart b/packages/flutter/test/material/time_picker_test.dart
new file mode 100644
index 0000000..82c24aa
--- /dev/null
+++ b/packages/flutter/test/material/time_picker_test.dart
@@ -0,0 +1,126 @@
+// 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/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('tap-select an hour', () {
+    testWidgets((WidgetTester tester) {
+      Key _timePickerKey = new UniqueKey();
+      TimeOfDay _selectedTime = const TimeOfDay(hour: 7, minute: 0);
+
+      tester.pumpWidget(
+        new StatefulBuilder(
+          builder: (BuildContext context, StateSetter setState) {
+            return new Material(
+              child: new Center(
+                  child: new SizedBox(
+                  width: 200.0,
+                  height: 400.0,
+                  child: new TimePicker(
+                    key: _timePickerKey,
+                    selectedTime: _selectedTime,
+                    onChanged: (TimeOfDay value) {
+                      setState(() {
+                        _selectedTime = value;
+                      });
+                    }
+                  )
+                )
+              )
+            );
+          }
+        )
+      );
+
+      Point center = tester.getCenter(find.byKey(_timePickerKey));
+
+      Point hour0 = new Point(center.x, center.y - 50.0); // 12:00 AM
+      tester.tapAt(hour0);
+      expect(_selectedTime.hour, equals(0));
+
+      Point hour3 = new Point(center.x + 50.0, center.y);
+      tester.tapAt(hour3);
+      expect(_selectedTime.hour, equals(3));
+
+      Point hour6 = new Point(center.x, center.y + 50.0);
+      tester.tapAt(hour6);
+      expect(_selectedTime.hour, equals(6));
+
+      Point hour9 = new Point(center.x - 50.0, center.y);
+      tester.tapAt(hour9);
+      expect(_selectedTime.hour, equals(9));
+
+      tester.pump(const Duration(seconds: 1)); // Finish gesture animation.
+      tester.pump(const Duration(seconds: 1)); // Finish settling animation.
+    });
+  });
+
+  test('drag-select an hour', () {
+    testWidgets((WidgetTester tester) {
+      Key _timePickerKey = new UniqueKey();
+      TimeOfDay _selectedTime = const TimeOfDay(hour: 7, minute: 0);
+
+      tester.pumpWidget(
+        new StatefulBuilder(
+          builder: (BuildContext context, StateSetter setState) {
+            return new Material(
+              child: new Center(
+                  child: new SizedBox(
+                  width: 200.0,
+                  height: 400.0,
+                  child: new TimePicker(
+                    key: _timePickerKey,
+                    selectedTime: _selectedTime,
+                    onChanged: (TimeOfDay value) {
+                      setState(() {
+                        _selectedTime = value;
+                      });
+                    }
+                  )
+                )
+              )
+            );
+          }
+        )
+      );
+
+      Point center = tester.getCenter(find.byKey(_timePickerKey));
+      Point hour0 = new Point(center.x, center.y - 50.0); // 12:00 AM
+      Point hour3 = new Point(center.x + 50.0, center.y);
+      Point hour6 = new Point(center.x, center.y + 50.0);
+      Point hour9 = new Point(center.x - 50.0, center.y);
+
+      tester.startGesture(hour3)
+        ..moveBy(hour0 - hour3)
+        ..up();
+      expect(_selectedTime.hour, equals(0));
+      tester.pump(const Duration(seconds: 1)); // Finish gesture animation.
+      tester.pump(const Duration(seconds: 1)); // Finish settling animation.
+
+      tester.startGesture(hour0)
+        ..moveBy(hour3 - hour0)
+        ..up();
+      expect(_selectedTime.hour, equals(3));
+      tester.pump(const Duration(seconds: 1));
+      tester.pump(const Duration(seconds: 1));
+
+      tester.startGesture(hour3)
+        ..moveBy(hour6 - hour3)
+        ..up();
+      expect(_selectedTime.hour, equals(6));
+      tester.pump(const Duration(seconds: 1));
+      tester.pump(const Duration(seconds: 1));
+
+      tester.startGesture(hour6)
+        ..moveBy(hour9 - hour6)
+        ..up();
+      expect(_selectedTime.hour, equals(9));
+      tester.pump(const Duration(seconds: 1));
+      tester.pump(const Duration(seconds: 1));
+    });
+  });
+}