title: Flutter DropdownButtonFormField does not show in Row
body: <p>I am adding a <code>DropdownFormField</code> and <code>TextFormField</code> side by side inside a Row. This is what the build function looks like</p>
  
  <pre><code>Widget build(BuildContext context) {
      return Form(
        key: _formKey,
        child: SingleChildScrollView(
          child: Column(
            children: &lt;Widget&gt;[
              Row(
                mainAxisAlignment: MainAxisAlignment.start,
                children: &lt;Widget&gt;[
                  DropdownButton(
                    hint: new Text('Select Gender'),
                    items: _salutations,
                    value: client.salutation,
                  ),
                  SizedBox(width: 20),
                  new Expanded(
                    child: TextFormField(
                      decoration: InputDecoration(labelText: 'Name'),
                      validator: (value) {
                        return value.isEmpty ? 'Empty name.' : '';
                      },
                    ),
                  ),
                ]
              )
            ],
          )
        ),
      );
    }
  </code></pre>
  
  <p>That code currently shows because it is DropdownButton. But if I change it to DropdownButtonFormField it won't show, as long as it is in the Row(). Why is this?</p>
  
  <p>I wish to use DropdownButtonFormField so that the height of the Dropdown will be the same as the TextFormField since the current code shows the dropdown with a smaller height.</p>