adds macOS implementation for battery plugin (#589)

experimental/federated_plugin/federated_plugin/ios/Classes/SwiftFederatedPlugin.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 Flutter
 import UIKit
 
@@ -9,7 +13,7 @@ public class SwiftFederatedPlugin: NSObject, FlutterPlugin {
   }
 
   public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
-    guard  call.method == "getBatteryLevel"  else {
+    guard call.method == "getBatteryLevel" else {
         result(FlutterMethodNotImplemented)
         return
     }