title: Flutter: DateTimeField manual editing problem
body: <p>I have <code>DateTimeField</code> and there is strange issue while editing the date manually. </p>
  
  <p>Here is the <strong>display with default value</strong>:</p>
  
  <p><a href="https://i.stack.imgur.com/9H0Va.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/9H0Va.png" alt="enter image description here"></a></p>
  
  <p>I selected month by double tapping and try to type <code>08</code> manually.</p>
  
  <p><a href="https://i.stack.imgur.com/KED3o.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/KED3o.png" alt="enter image description here"></a></p>
  
  <p>When I bring pointer at the end of month <code>12</code>, and pressed <code>backspace</code> to remove <code>2</code> from <code>12</code>. The month was changed to <code>01</code>. </p>
  
  <p><a href="https://i.stack.imgur.com/3xTee.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/3xTee.png" alt="enter image description here"></a></p>
  
  <p>When I press <code>backspace</code> in the end of year, to remove <code>8</code> from <code>2018</code>. It was changed to <code>0201</code>.</p>
  
  <p><a href="https://i.stack.imgur.com/w2aGL.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/w2aGL.png" alt="enter image description here"></a></p>
  
  <p>Here is the <strong>code</strong> of that field:</p>
  
  <pre><code>DateTimeField(
  format: DateFormat("yyyy-MM-dd hh:mm:ss"),
  onSaved: (val) =&gt; setState(() =&gt; _fromDate = val),
  keyboardType: TextInputType.datetime,
  onChanged: (DateTime newValue) {
    setState(() {
      _fromDate = newValue;
    });
  },
  onShowPicker: (context, currentValue) {
    return showDatePicker(
        context: context,
        firstDate: DateTime.now(),
        initialDate: currentValue ?? DateTime.now(),
        lastDate: DateTime.now().add(new Duration(days: 30))
        );
  },
  );
  </code></pre>
  
  <p>I have no clue, what's going on with this. Please tell me, what could be wrong?</p>
  
  <p><strong>NOTE:</strong></p>
  
  <ol>
  <li>Using picker for date selection works fine</li>
  <li>I've <code>date</code> field in another page, it's in <code>yyyy-MM-dd</code> format, and it works as expected there.</li>
  </ol>