Upgrading Flutter to Flutter 1.9 breaks xcode build

[JacobT14]

Hello!

After upgrading my Flutter app to Flutter 1.9, this package now fails to build due to the following error:

Failed to build iOS app
Error output from Xcode build:
↳
    ** BUILD FAILED **

Xcode's output:
↳
    === BUILD TARGET sqflite OF PROJECT Pods WITH CONFIGURATION Debug ===
    In file included from /Users/jft/projects/flutter/.pub-cache/hosted/pub.dartlang.org/barcode_scan-1.0.0/ios/Classes/BarcodeScanPlugin.m:1:
    /Users/jft/projects/flutter/.pub-cache/hosted/pub.dartlang.org/barcode_scan-1.0.0/ios/Classes/BarcodeScanPlugin.h:12:1: warning: retain'ed block property does not copy the block - use copy attribute instead [-Wobjc-noncopy-retain-block-property]
    @property(nonatomic, retain) FlutterResult result;
    ^
    1 warning generated.
    === BUILD TARGET Runner OF PROJECT Runner WITH CONFIGURATION Debug ===
    The use of Swift 3 @objc inference in Swift 4 mode is deprecated. Please address deprecated @objc inference warnings, test your code with “Use of deprecated Swift 3 @objc inference” logging enabled, and then disable inference by changing the "Swift 3 @objc Inference" build setting to "Default" for the "Runner" target.
    === BUILD TARGET Runner OF PROJECT Runner WITH CONFIGURATION Debug ===
    /Users/jft/projects/hive-mobile-flutter/ios/Runner/AppDelegate.swift:18:34: error: argument type 'FlutterViewController' does not conform to expected type 'FlutterBinaryMessenger'
                    binaryMessenger: controller, codec: FlutterJSONMethodCodec.sharedInstance());
                                     ^~~~~~~~~~
                                                as! FlutterBinaryMessenger

Seems it should be an easy fix! Let me know if you want a PR or if you can take care of it!

Thanks!