Fix asserts for initialDateTime in CupertinoDatePicker (#27272)

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

diff --git a/packages/flutter/lib/src/cupertino/date_picker.dart b/packages/flutter/lib/src/cupertino/date_picker.dart
index fc6d05a..069358a 100644
--- a/packages/flutter/lib/src/cupertino/date_picker.dart
+++ b/packages/flutter/lib/src/cupertino/date_picker.dart
@@ -168,7 +168,6 @@
   CupertinoDatePicker({
     this.mode = CupertinoDatePickerMode.dateAndTime,
     @required this.onDateTimeChanged,
-    // ignore: always_require_non_null_named_parameters
     DateTime initialDateTime,
     this.minimumDate,
     this.maximumDate,
@@ -179,32 +178,33 @@
   }) : initialDateTime = initialDateTime ?? DateTime.now(),
        assert(mode != null),
        assert(onDateTimeChanged != null),
-       assert(initialDateTime != null),
-       assert(
-         mode != CupertinoDatePickerMode.dateAndTime || minimumDate == null || !initialDateTime.isBefore(minimumDate),
-         'initial date is before minimum date',
-       ),
-       assert(
-         mode != CupertinoDatePickerMode.dateAndTime || maximumDate == null || !initialDateTime.isAfter(maximumDate),
-         'initial date is after maximum date',
-       ),
        assert(minimumYear != null),
        assert(
-         mode != CupertinoDatePickerMode.date || (minimumYear >= 1 && initialDateTime.year >= minimumYear),
-         'initial year is not greater than minimum year, or mininum year is not positive',
-       ),
-       assert(
-         mode != CupertinoDatePickerMode.date || maximumYear == null || initialDateTime.year <= maximumYear,
-         'initial year is not smaller than maximum year',
-       ),
-       assert(
          minuteInterval > 0 && 60 % minuteInterval == 0,
          'minute interval is not a positive integer factor of 60',
-       ),
-       assert(
-         initialDateTime.minute % minuteInterval == 0,
-         'initial minute is not divisible by minute interval',
-       );
+       ) {
+    assert(this.initialDateTime != null);
+    assert(
+      mode != CupertinoDatePickerMode.dateAndTime || minimumDate == null || !this.initialDateTime.isBefore(minimumDate),
+      'initial date is before minimum date',
+    );
+    assert(
+      mode != CupertinoDatePickerMode.dateAndTime || maximumDate == null || !this.initialDateTime.isAfter(maximumDate),
+      'initial date is after maximum date',
+    );
+    assert(
+      mode != CupertinoDatePickerMode.date || (minimumYear >= 1 && this.initialDateTime.year >= minimumYear),
+      'initial year is not greater than minimum year, or mininum year is not positive',
+    );
+    assert(
+      mode != CupertinoDatePickerMode.date || maximumYear == null || this.initialDateTime.year <= maximumYear,
+      'initial year is not smaller than maximum year',
+    );
+    assert(
+      this.initialDateTime.minute % minuteInterval == 0,
+      'initial minute is not divisible by minute interval',
+    );
+  }
 
   /// The mode of the date picker as one of [CupertinoDatePickerMode].
   /// Defaults to [CupertinoDatePickerMode.dateAndTime]. Cannot be null and

packages/flutter/test/cupertino/date_picker_test.dart

diff --git a/packages/flutter/test/cupertino/date_picker_test.dart b/packages/flutter/test/cupertino/date_picker_test.dart
index 6491552..027d687 100644
--- a/packages/flutter/test/cupertino/date_picker_test.dart
+++ b/packages/flutter/test/cupertino/date_picker_test.dart
@@ -221,28 +221,11 @@
       );
     });
 
-    testWidgets('initial date time is not null', (WidgetTester tester) async {
-      expect(
-        () {
-          CupertinoDatePicker(
-            onDateTimeChanged: (_) {},
-            initialDateTime: null,
-          );
-        },
-        throwsAssertionError,
+    testWidgets('initial date is set to default value', (WidgetTester tester) async {
+      final CupertinoDatePicker picker = CupertinoDatePicker(
+        onDateTimeChanged: (_) {},
       );
-    });
-
-    testWidgets('initial date time is not null', (WidgetTester tester) async {
-      expect(
-            () {
-          CupertinoDatePicker(
-            onDateTimeChanged: (_) {},
-            initialDateTime: null,
-          );
-        },
-        throwsAssertionError,
-      );
+      expect(picker.initialDateTime, isNotNull);
     });
 
     testWidgets('changing initialDateTime after first build does not do anything', (WidgetTester tester) async {