flutter之CocoaPods could not find

运行 flutter build ios 的时候报错Error running pod install，中间的报错信息如下：

[!] CocoaPods could not find compatible versions for
    pod "xxPlugin":
      In Podfile:
        FlutterPluginRegistrant (from
        `Flutter/FlutterPluginRegistrant`) was resolved to
        0.0.1, which depends on
          xxPlugin

        xxPlugin (from
        `Flutter/.symlinks/xxPlugin/ios`)

因为 flutter build ios 的时候会运行 pod install 命令，而 ios（或者.ios）下的 PodFile文件如下

platform :ios, '8.0'

flutter_application_path = '../'
load File.join(flutter_application_path, '.ios', 'Flutter', 'podhelper.rb')

use_frameworks!

target 'Runner' do
  install_flutter_engine_pod
  install_flutter_plugin_pods flutter_application_path
end

会根据 path : ios/Flutter/FlutterPluginRegistrant/ 本地引用FlutterPluginRegistrant。

而 FlutterPluginRegistrant.podspec 内会引用所有的 plugin（有ios和android代码）

#
# Generated file, do not edit.
#

Pod::Spec.new do |s|
  s.name             = 'FlutterPluginRegistrant'
  s.version          = '0.0.1'
  s.summary          = 'Registers plugins with your flutter app'
  s.description      = <<-DESC
Depends on all your plugins, and provides a function to register them.
                       DESC
  s.homepage         = 'https://flutter.dev'
  s.license          = { :type => 'BSD' }
  s.author           = { 'Flutter Dev Team' => 'flutter-dev@googlegroups.com' }
  s.ios.deployment_target = '8.0'
  s.source_files =  "Classes", "Classes/**/*.{h,m}"
  s.source           = { :path => '.' }
  s.public_header_files = './Classes/**/*.h'
  s.static_framework    = true
  s.pod_target_xcconfig = { 'DEFINES_MODULE' => 'YES' }
  s.dependency 'Flutter'
  s.dependency 'bridgePlugin'
  s.dependency 'flutter_boost'
end

查看下所有的 plugin 的 podspec 中指定的 ios 版本要求，是否高于 ios 文件夹下 PodFile 的版本，如果高于，修改即可。