title: showDatePicker's header overflow error
body: <p>Hello I'm having an issue with the date picker in flutter, when calling it from within a stepper's step:</p>
  
  <p><a href="https://i.stack.imgur.com/ODQOY.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ODQOY.png" alt="enter image description here"></a></p>
  
  <p>I'm calling the showDateTimePicker from this step:</p>
  
  <pre><code> new Step(
        title: new Text(
          'Día',
          style: TextStyle(
            color: Theme.of(context).primaryColor,
          ),
        ),
        content: Container(
          height: MediaQuery.of(context).size.width - 100.0,
          child: ListView(
            children: &lt;Widget&gt;[
              DateTimePickerFormField(
                style: TextStyle(
                  color: Theme.of(context).primaryColor,
                  fontSize: 25.0,
                ),
                format: DateFormat('dd/MM/yyyy - hh:mm'),
                onChanged: (date) {
                  if (date != null) print(date.toIso8601String());
                },
              ),
            ],
          ),
        ),
        isActive: _currentStep &gt;= 0,
        state: _currentStep &gt;= 0 ? StepState.complete : StepState.disabled,
      ),
  </code></pre>
  
  <p>this is the error stack:</p>
  
  <pre><code>I/flutter ( 4388): The specific RenderFlex in question is:
  I/flutter ( 4388):   RenderFlex#26a8f OVERFLOWING
  I/flutter ( 4388):   creator: Column ← Padding ← DecoratedBox ← ConstrainedBox ← Container ← _DatePickerHeader ← Column
  I/flutter ( 4388):   ← SizedBox ← LayoutBuilder ← OrientationBuilder ← DefaultTextStyle ← AnimatedDefaultTextStyle ← ⋯
  I/flutter ( 4388):   parentData: offset=Offset(16.0, 0.0) (can use size)
  I/flutter ( 4388):   constraints: BoxConstraints(w=298.0, h=100.0)
  I/flutter ( 4388):   size: Size(298.0, 100.0)
  I/flutter ( 4388):   direction: vertical
  I/flutter ( 4388):   mainAxisAlignment: center
  I/flutter ( 4388):   mainAxisSize: max
  I/flutter ( 4388):   crossAxisAlignment: start
  I/flutter ( 4388):   textDirection: ltr
  I/flutter ( 4388):   verticalDirection: down
  </code></pre>
  
  <p>Maybe someone sorted out this?</p>