Fix example (#166)

* example: Sort imports.

* example: Remove space.

* example: Use ElevatedButton instead of RaisedButton.

* example: Lint.

Author: ryojiro

example/lib/main.dart

@@ -1,10 +1,9 @@
 import 'dart:async';
 import 'dart:io';
 
+import 'package:callkeep/callkeep.dart';
 import 'package:firebase_messaging/firebase_messaging.dart';
 import 'package:flutter/material.dart';
-
-import 'package:callkeep/callkeep.dart';
 import 'package:uuid/uuid.dart';
 
 /// For fcm background message handler.
@@ -16,7 +15,7 @@ bool _callKeepInited = false;
     "uuid": "xxxxx-xxxxx-xxxxx-xxxxx",
     "caller_id": "+8618612345678",
     "caller_name": "hello",
-    "caller_id_type": "number", 
+    "caller_id_type": "number",
     "has_video": false,
 
     "extra": {
@@ -377,25 +376,25 @@ class _MyAppState extends State<HomePage> {
                   Row(
                     mainAxisAlignment: MainAxisAlignment.center,
                     children: <Widget>[
-                      RaisedButton(
+                      ElevatedButton(
                         onPressed: () async {
                           setOnHold(item.key, !item.value.held);
                         },
                         child: Text(item.value.held ? 'Unhold' : 'Hold'),
                       ),
-                      RaisedButton(
+                      ElevatedButton(
                         onPressed: () async {
                           updateDisplay(item.key);
                         },
                         child: const Text('Display'),
                       ),
-                      RaisedButton(
+                      ElevatedButton(
                         onPressed: () async {
                           setMutedCall(item.key, !item.value.muted);
                         },
                         child: Text(item.value.muted ? 'Unmute' : 'Mute'),
                       ),
-                      RaisedButton(
+                      ElevatedButton(
                         onPressed: () async {
                           hangup(item.key);
                         },
@@ -418,13 +417,13 @@ class _MyAppState extends State<HomePage> {
           child: Column(
             mainAxisAlignment: MainAxisAlignment.start,
             children: [
-              RaisedButton(
+              ElevatedButton(
                 onPressed: () async {
                   displayIncomingCall('10086');
                 },
                 child: const Text('Display incoming call now'),
               ),
-              RaisedButton(
+              ElevatedButton(
                 onPressed: () async {
                   displayIncomingCallDelayed('10086');
                 },

example/test/widget_test.dart

@@ -6,9 +6,8 @@
 // tree, read text, and verify that the values of widget properties are correct.
 
 import 'package:flutter/material.dart';
-import 'package:flutter_test/flutter_test.dart';
-
 import 'package:flutter_callkeep_example/main.dart';
+import 'package:flutter_test/flutter_test.dart';
 
 void main() {
   testWidgets('Verify Platform version', (WidgetTester tester) async {
@@ -18,8 +17,8 @@ void main() {
     // Verify that platform version is retrieved.
     expect(
       find.byWidgetPredicate(
-        (Widget widget) => widget is Text &&
-                           widget.data.startsWith('Running on:'),
+        (Widget widget) =>
+            widget is Text && widget.data.startsWith('Running on:'),
       ),
       findsOneWidget,
     );