[platform_channels] adds EventChannel implementation for iOS (#595)

platform_channels/ios/Runner/AppDelegate.swift

@@ -1,3 +1,7 @@
+// Copyright 2020 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 import UIKit
 import Flutter
 
@@ -39,6 +43,8 @@ import Flutter
             }
         }
         
+        FlutterEventChannel(name: "eventChannelDemo", binaryMessenger: flutterViewController.binaryMessenger).setStreamHandler(AccelerometerStreamHandler())
+        
         GeneratedPluginRegistrant.register(with: self)
         return super.application(application, didFinishLaunchingWithOptions: launchOptions)
     }