Implement CupertinoDatepicker time/dateTime constraints (#44628)

packages/flutter/test/cupertino/date_picker_test.dart

@@ -661,7 +661,7 @@ void main() {
     });
 
     testWidgets(
-      'picker automatically scrolls away from invalid date, '
+      'date picker automatically scrolls away from invalid date, '
       "and onDateTimeChanged doesn't report these dates",
       (WidgetTester tester) async {
         DateTime date;
@@ -727,6 +727,160 @@ void main() {
         );
     });
 
+    testWidgets(
+      'dateTime picker automatically scrolls away from invalid date, '
+      "and onDateTimeChanged doesn't report these dates",
+      (WidgetTester tester) async {
+        DateTime date;
+        final DateTime minimum = DateTime(2019, 11, 11, 3, 30);
+        final DateTime maximum = DateTime(2019, 11, 11, 14, 59, 59);
+        await tester.pumpWidget(
+          CupertinoApp(
+            home: Center(
+              child: SizedBox(
+                height: 400.0,
+                width: 400.0,
+                child: CupertinoDatePicker(
+                  mode: CupertinoDatePickerMode.dateAndTime,
+                  minimumDate: minimum,
+                  maximumDate: maximum,
+                  onDateTimeChanged: (DateTime newDate) {
+                    date = newDate;
+                    // Callback doesn't transiently go into invalid dates.
+                    expect(minimum.isAfter(newDate), isFalse);
+                    expect(maximum.isBefore(newDate), isFalse);
+                  },
+                  initialDateTime: DateTime(2019, 11, 11, 4),
+                ),
+              ),
+            ),
+          ),
+        );
+
+        // 3:00 is valid but 2:00 should be invalid.
+        expect(
+          tester.widget<Text>(find.text('3')).style.color,
+          isNot(isSameColorAs(CupertinoColors.inactiveGray.color)),
+        );
+
+        expect(
+          tester.widget<Text>(find.text('2')).style.color,
+          isSameColorAs(CupertinoColors.inactiveGray.color),
+        );
+
+        // 'PM' is greyed out.
+        expect(
+          tester.widget<Text>(find.text('PM')).style.color,
+          isSameColorAs(CupertinoColors.inactiveGray.color),
+        );
+
+        await tester.drag(find.text('AM'), const Offset(0.0, -32.0), touchSlopY: 0.0);
+        await tester.pump();
+        await tester.pumpAndSettle(); // Now the autoscrolling should happen.
+
+        expect(
+          date,
+          DateTime(2019, 11, 11, 14, 59),
+        );
+
+        // 3'o clock and 'AM' are now greyed out.
+        expect(
+          tester.widget<Text>(find.text('AM')).style.color,
+          isSameColorAs(CupertinoColors.inactiveGray.color),
+        );
+        expect(
+          tester.widget<Text>(find.text('3')).style.color,
+          isSameColorAs(CupertinoColors.inactiveGray.color),
+        );
+
+        await tester.drag(find.text('PM'), const Offset(0.0, 32.0), touchSlopY: 0.0);
+        await tester.pump(); // Once to trigger the post frame animate call.
+        await tester.pumpAndSettle();
+
+        // Returns to min date.
+        expect(
+          date,
+          DateTime(2019, 11, 11, 3, 30),
+        );
+    });
+
+    testWidgets(
+      'time picker automatically scrolls away from invalid date, '
+      "and onDateTimeChanged doesn't report these dates",
+      (WidgetTester tester) async {
+        DateTime date;
+        final DateTime minimum = DateTime(2019, 11, 11, 3, 30);
+        final DateTime maximum = DateTime(2019, 11, 11, 14, 59, 59);
+        await tester.pumpWidget(
+          CupertinoApp(
+            home: Center(
+              child: SizedBox(
+                height: 400.0,
+                width: 400.0,
+                child: CupertinoDatePicker(
+                  mode: CupertinoDatePickerMode.time,
+                  minimumDate: minimum,
+                  maximumDate: maximum,
+                  onDateTimeChanged: (DateTime newDate) {
+                    date = newDate;
+                    // Callback doesn't transiently go into invalid dates.
+                    expect(minimum.isAfter(newDate), isFalse);
+                    expect(maximum.isBefore(newDate), isFalse);
+                  },
+                  initialDateTime: DateTime(2019, 11, 11, 4),
+                ),
+              ),
+            ),
+          ),
+        );
+
+        // 3:00 is valid but 2:00 should be invalid.
+        expect(
+          tester.widget<Text>(find.text('3')).style.color,
+          isNot(isSameColorAs(CupertinoColors.inactiveGray.color)),
+        );
+
+        expect(
+          tester.widget<Text>(find.text('2')).style.color,
+          isSameColorAs(CupertinoColors.inactiveGray.color),
+        );
+
+        // 'PM' is greyed out.
+        expect(
+          tester.widget<Text>(find.text('PM')).style.color,
+          isSameColorAs(CupertinoColors.inactiveGray.color),
+        );
+
+        await tester.drag(find.text('AM'), const Offset(0.0, -32.0), touchSlopY: 0.0);
+        await tester.pump();
+        await tester.pumpAndSettle(); // Now the autoscrolling should happen.
+
+        expect(
+          date,
+          DateTime(2019, 11, 11, 14, 59),
+        );
+
+        // 3'o clock and 'AM' are now greyed out.
+        expect(
+          tester.widget<Text>(find.text('AM')).style.color,
+          isSameColorAs(CupertinoColors.inactiveGray.color),
+        );
+        expect(
+          tester.widget<Text>(find.text('3')).style.color,
+          isSameColorAs(CupertinoColors.inactiveGray.color),
+        );
+
+        await tester.drag(find.text('PM'), const Offset(0.0, 32.0), touchSlopY: 0.0);
+        await tester.pump(); // Once to trigger the post frame animate call.
+        await tester.pumpAndSettle();
+
+        // Returns to min date.
+        expect(
+          date,
+          DateTime(2019, 11, 11, 3, 30),
+        );
+    });
+
     testWidgets('picker automatically scrolls away from invalid date on day change', (WidgetTester tester) async {
       DateTime date;
       await tester.pumpWidget(