title: The prefixIcon of TextField widget in flutter disappears on focusing TextField. How to solve?
body: <p>Was just exploring flutter and got stuck. The prefixIcon disappears on clicking field and it appears back when focused out. How to solve this ? The code is as below. Tried removing the key of form. I want the icon to stay even if focused in or focused out. Is there any other property to set? 
  Couldn't be able to find a fix.</p>
  
  <pre><code>        class _MyHomePageState extends State&lt;MyHomePage&gt; {
            String _email = "";
            String _password = "";
  
            final  _formKey = GlobalKey&lt;FormState&gt;();
            final FocusNode _emailFocus = FocusNode();
            final FocusNode _passwordFocus = FocusNode();
  
  
  
            @override
            Widget build(BuildContext context) {
              return Scaffold(
                appBar: AppBar(
                  title: Text(widget.title),
                ),
                body: Container(
                        padding: const EdgeInsets.symmetric(horizontal: 50.0,vertical: 100.0),
                        decoration: BoxDecoration(color: Colors.white),
                        child:Form(
                          key: _formKey,
                          child: Column(
                            crossAxisAlignment: CrossAxisAlignment.start,
                            children: &lt;Widget&gt;[
                              TextFormField(
                                focusNode: _emailFocus,
                                  decoration: InputDecoration(
                                      labelText: 'Username or email',
                                      prefixIcon: Icon(Icons.person), //prefixIcon
                                    focusedBorder: UnderlineInputBorder(),
                                    hintText: "example@mail.com",
                                  )
                              ),
                              Padding(
                                padding: const EdgeInsets.symmetric(vertical: 16.0),
                              ),
                                TextFormField(
                                obscureText: true,
                                focusNode: _passwordFocus,
                                    decoration: InputDecoration(
                                      labelText: 'Password',
                                      prefixIcon: Icon(Icons.lock),
                                      focusedBorder: UnderlineInputBorder(),
                                    )
                              ) ,
                              Padding(
                                padding: const EdgeInsets.symmetric(vertical: 16.0),
                                child: RaisedButton(
                                  onPressed: () {
                                    // Validate will return true if the form is valid, or false if
                                    // the form is invalid.
                                    if (_formKey.currentState.validate()) {
                                      // Process data.
                                    }
                                  },
                                  child: Text('Submit'),
                                ),
                              ),
                            ],
                          ),
                        )
                )
              );
            }
          }
  </code></pre>