Repository: flutter
Issue: How to unfold DropdownButton menu only downward
Labels: c: new feature,framework,f: material design,P3,team-design,triaged-design

This is my code for the DropdownButton menu:
```dart
import 'package:flutter/material.dart';
import 'package:percent_indicator/percent_indicator.dart';
import 'package:crypti/main.dart';


// TODO: Change place of Next button since keyboard will appear in screen normally
// TODO: force keyboard in screen (also in simulator)
void main() {
  runApp(MaterialApp(
    theme:
    ThemeData(accentColor: Colors.black87),
    home: ThirdRoute(),
  ));
}

class ThirdRoute extends StatefulWidget {
  @override
  _ThirdRouteState createState() => new _ThirdRouteState();
}

class Coin {
  int id;
  String name;

  Coin(this.id, this.name);

  static List<Coin> getCoins() {
    return <Coin>[
      Coin(1, 'Bitcoin'),
      Coin(2, 'Ethereum'),
      Coin(3, 'Ripple'),
      Coin(4, 'Litecoin'),
      Coin(5, 'EOS'),
    ];
  }
}


class _ThirdRouteState extends State<ThirdRoute> {

  List<Coin> _coins = Coin.getCoins();
  List<DropdownMenuItem<Coin>> _dropdownMenuItems;
  Coin _selectedCoin;

  @override
  void initState() {
    _dropdownMenuItems = buildDropdownMenuItems(_coins);
    _selectedCoin = _dropdownMenuItems[0].value;
    super.initState();
  }

  List<DropdownMenuItem<Coin>> buildDropdownMenuItems(List coins) {
    List<DropdownMenuItem<Coin>> items = List();
    for (Coin coin in coins) {
      items.add(
        DropdownMenuItem(
          value: coin,
          child:
          Text(
            coin.name,
            style: TextStyle(
                fontSize: 18.0,
                color: Colors.black87,
                fontWeight: FontWeight.bold
            ),
          ),
        ),
      );
    }
    return items;
  }

  onChangeDropdownItem(Coin selectedCoin) {
    setState(() {
      _selectedCoin = selectedCoin;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Stack(
        fit: StackFit.expand,
        children: <Widget>[
          Container(
            decoration: BoxDecoration(
                color: Color.fromRGBO(34, 139, 34, 0.4)),
          ),
          Positioned(
            top: 0.0,
            left: 0.0,
            right: 0.0,
            child: AppBar(
              backgroundColor: Colors.transparent,
              elevation: 0.0,
            ),
          ),

          Column(
            crossAxisAlignment: CrossAxisAlignment.start,
            children: <Widget>[
              Padding(
                padding: EdgeInsets.symmetric(vertical: 50.0),
              ),
              Padding(
                padding: EdgeInsets.fromLTRB(20.0,16.0,0.0,0.0), // left, top, right, bottom
                child: LinearPercentIndicator(
                  width: MediaQuery.of(context).size.width - 50,
                  animation: true,
                  lineHeight: 20.0,
                  animationDuration: 2000,
                  percent: 0.4,
                  center: Text("40.0%"),
                  linearStrokeCap: LinearStrokeCap.roundAll,
                  progressColor: Colors.greenAccent,
                ),
              ),
              Padding(
                padding: EdgeInsets.fromLTRB(40.0,16.0,0.0,0.0),
                child: Text("Select the coin you want to be paid in",
                  style: TextStyle(
                      fontSize: 35.0,
                      color: Colors.white,
                      fontWeight: FontWeight.w400
                  ),
                ),
              ),
              Padding(
                padding: EdgeInsets.fromLTRB(160.0,0.0,0.0,12.0),
                child: DropdownButton(
                  value: _selectedCoin,
                  items: _dropdownMenuItems,
                  onChanged: onChangeDropdownItem,
                ),
              ),
              Text('Selected: ${_selectedCoin.name}'),
            ],
          ),
        ],
      ),
    );
  }
}
```

Instead of having the DropdownButton menu unfolding both up and down, I would like to have it unfold only downward so it doesn't overlap the title on my screen. Current situation:
![Screenshot 2019-08-30 at 11 19 28](https://user-images.githubusercontent.com/47478429/64009461-30ce7080-cb18-11e9-8bcb-3e0efd7cdcbc.png)