title: Getting error with CupertinoDatePicker: initial minute is not divisible by minute interval
body: <p>Trying to have a simple <code>CupertinoDatePicker</code>. When I set <code>minuteInterval</code>, I get an error:</p>
  <blockquote>
  <p>The following assertion was thrown building TimePicker(dirty):
  initial minute is not divisible by minute interval</p>
  </blockquote>
  <pre><code>CupertinoDatePicker(
        onDateTimeChanged: (DateTime newDate) =&gt; print('changed'),
        mode: CupertinoDatePickerMode.time,
        initialDateTime: DateTime.now(),
        minuteInterval: 5,
      )
  </code></pre>
  <p>Seems like the minutes for DateTime.now() has to be divisible by 5.</p>
  <p>How do I make sure the times minutes either end with 0 or 5 whichever is later?
  so say it's 7:42,  the minute should be 45. If the minute is 47, it should be 50?</p>