title: Flutter DropdownButton show label when option is selected
body: <p>Can a <code>Dropdown Button</code>: </p>
  
  <pre><code> return DropdownButton&lt;String&gt;(
            items: &lt;String&gt;['Foo', 'Bar'].map((String value) {
              return new DropdownMenuItem&lt;String&gt;(
                value: value,
                child: new Text(value),
              );
            }).toList(),
            onChanged: (_) {},
          );
  </code></pre>
  
  <p>have something similar to a decoration user in a <code>TextFormField</code>: </p>
  
  <pre><code>      TextFormField(
          controller: _titleController,
          decoration: InputDecoration(labelText: 'Input'),
          validator: (String value) {
            if (value != null &amp;&amp; value.isEmpty) {
              return 'Please enter some text';
            }
          },
          style: Theme.of(context).textTheme.title,
        ),
  </code></pre>
  
  <p>When something is written in the above TextFormField the word <code>Input</code> shows. Like this: </p>
  
  <p><a href="https://i.stack.imgur.com/ro42r.png" rel="noreferrer"><img src="https://i.stack.imgur.com/ro42r.png" alt="enter image description here"></a></p>