Fix dart future not returning on cancel

ios/Classes/BarcodeScanPlugin.h

@@ -9,6 +9,6 @@
 
 @interface BarcodeScanPlugin : NSObject<FlutterPlugin, BarcodeScannerViewControllerDelegate>
 
-@property(nonatomic, retain) FlutterResult result;
+@property(nonatomic, copy) FlutterResult result;
 @property (nonatomic, assign) UIViewController *hostViewController;
 @end

ios/Classes/BarcodeScannerViewController.m

@@ -82,6 +82,7 @@ - (void)startScan {
 }
 
 - (void)cancel {
+    [self.delegate barcodeScannerViewController:self didFailWithErrorCode:@"USER_CANCELED"];
     [self dismissViewControllerAnimated:true completion:nil];
 }
 

lib/barcode_scan.dart

@@ -4,6 +4,7 @@ import 'package:flutter/services.dart';
 
 class BarcodeScanner {
   static const CameraAccessDenied = 'PERMISSION_NOT_GRANTED';
+  static const UserCanceled = 'USER_CANCELED';
   static const MethodChannel _channel =
       const MethodChannel('com.apptreesoftware.barcode_scan');
   static Future<String> scan() async => await _channel.invokeMethod('scan');