refs #60: expose state to let use a key to open the dialog

lib/country_code_picker.dart

@@ -78,7 +78,8 @@ class CountryCodePicker extends StatefulWidget {
     this.comparator,
     this.customList,
     this.dialogSize,
-  });
+    Key key,
+  }) : super(key: key);
 
   @override
   State<StatefulWidget> createState() {
@@ -104,29 +105,29 @@ class CountryCodePicker extends StatefulWidget {
       elements = elements.where((c) => countryFilter.contains(c.code)).toList();
     }
 
-    return _CountryCodePickerState(elements);
+    return CountryCodePickerState(elements);
   }
 }
 
-class _CountryCodePickerState extends State<CountryCodePicker> {
+class CountryCodePickerState extends State<CountryCodePicker> {
   CountryCode selectedItem;
   List<CountryCode> elements = [];
   List<CountryCode> favoriteElements = [];
 
-  _CountryCodePickerState(this.elements);
+  CountryCodePickerState(this.elements);
 
   @override
   Widget build(BuildContext context) {
     Widget _widget;
     if (widget.builder != null)
       _widget = InkWell(
-        onTap: _showSelectionDialog,
+        onTap: showCountryCodePickerDialog,
         child: widget.builder(selectedItem.localize(context)),
       );
     else {
       _widget = FlatButton(
         padding: widget.padding,
-        onPressed: widget.enabled ? _showSelectionDialog : null,
+        onPressed: widget.enabled ? showCountryCodePickerDialog : null,
         child: Flex(
           direction: Axis.horizontal,
           mainAxisSize: MainAxisSize.min,
@@ -205,7 +206,7 @@ class _CountryCodePickerState extends State<CountryCodePicker> {
     super.initState();
   }
 
-  void _showSelectionDialog() {
+  void showCountryCodePickerDialog() {
     showDialog(
       context: context,
       builder: (_) => SelectionDialog(