CupertinoDatePicker initialDateTime accounts for minuteInterval (#30862)

initialDateTime should account for minuteInterval

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

@@ -540,7 +540,7 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
 
   Widget _buildMinutePicker(double offAxisFraction, TransitionBuilder itemPositioningBuilder) {
     return CupertinoPicker(
-      scrollController: FixedExtentScrollController(initialItem: selectedMinute),
+      scrollController: FixedExtentScrollController(initialItem: selectedMinute ~/ widget.minuteInterval),
       offAxisFraction: offAxisFraction,
       itemExtent: _kItemExtent,
       useMagnifier: _kUseMagnifier,

packages/flutter/test/cupertino/date_picker_test.dart

@@ -266,6 +266,31 @@ void main() {
       expect(picker.initialDateTime, isNotNull);
     });
 
+    testWidgets('initial date honors minuteInterval', (WidgetTester tester) async {
+        DateTime newDateTime;
+        await tester.pumpWidget(
+          CupertinoApp(
+            home: SizedBox(
+              width: 400,
+              height: 400,
+              child: CupertinoDatePicker(
+                onDateTimeChanged: (DateTime d) => newDateTime = d,
+                initialDateTime: DateTime(2018, 10, 10, 10, 3),
+                minuteInterval: 3,
+              )
+            )
+          )
+        );
+
+        // Drag the minute picker to the next slot (03 -> 06).
+        // The `initialDateTime` and the `minuteInterval` values are specifically choosen
+        // so that `find.text` finds exactly one widget.
+        await tester.drag(find.text('03'), _kRowOffset);
+        await tester.pump();
+
+        expect(newDateTime.minute, 6);
+    });
+
     testWidgets('changing initialDateTime after first build does not do anything', (WidgetTester tester) async {
       DateTime selectedDateTime;
       await tester.pumpWidget(