Disable DatePicker vibrate on date change on iOS (#8335)

In the Android native material DatePicker, haptic feedback is expected
on date change. This is not expected behaviour for iOS date/time
pickers.

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

@@ -591,6 +591,8 @@ class _DatePickerDialog extends StatefulWidget {
 }
 
 class _DatePickerDialogState extends State<_DatePickerDialog> {
+  bool _vibrate = false;
+
   @override
   void initState() {
     super.initState();
@@ -602,6 +604,7 @@ class _DatePickerDialogState extends State<_DatePickerDialog> {
   GlobalKey _pickerKey = new GlobalKey();
 
   void _handleModeChanged(_DatePickerMode mode) {
+    if (_vibrate)
       HapticFeedback.vibrate();
     setState(() {
       _mode = mode;
@@ -609,6 +612,7 @@ class _DatePickerDialogState extends State<_DatePickerDialog> {
   }
 
   void _handleYearChanged(DateTime value) {
+    if (_vibrate)
       HapticFeedback.vibrate();
     setState(() {
       _mode = _DatePickerMode.day;
@@ -617,6 +621,7 @@ class _DatePickerDialogState extends State<_DatePickerDialog> {
   }
 
   void _handleDayChanged(DateTime value) {
+    if (_vibrate)
       HapticFeedback.vibrate();
     setState(() {
       _selectedDate = value;
@@ -657,6 +662,15 @@ class _DatePickerDialogState extends State<_DatePickerDialog> {
 
   @override
   Widget build(BuildContext context) {
+    switch (Theme.of(context).platform) {
+      case TargetPlatform.android:
+      case TargetPlatform.fuchsia:
+        _vibrate = true;
+        break;
+      case TargetPlatform.iOS:
+        _vibrate = false;
+        break;
+    }
     Widget picker = new Flexible(
       child: new SizedBox(
         height: _kMaxDayPickerHeight,