1.运行$ flutter doctor 提示

[✓] Flutter (Channel stable, v1.7.8+hotfix.4, on Mac OS X 10.14.5 18F203, locale
    zh-Hans-CN)
[!] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
    ✗ Android license status unknown.
      Try re-installing or updating your Android SDK Manager.
      See https://developer.android.com/studio/#downloads or visit
      https://flutter.dev/setup/#android-setup for detailed instructions.
[✗] Xcode - develop for iOS and macOS
    ✗ Xcode installation is incomplete; a full installation is necessary for iOS
      development.
      Download at: https://developer.apple.com/xcode/download/
      Or install Xcode via the App Store.
      Once installed, run:
        sudo xcode-select --switch /Applications/Xcode.app/Contents/Developer
    ✗ CocoaPods not installed.
        CocoaPods is used to retrieve the iOS and macOS platform side's plugin
        code that responds to your plugin usage on the Dart side.
        Without CocoaPods, plugins will not work on iOS or macOS.
        For more info, see https://flutter.dev/platform-plugins
      To install:
        brew install cocoapods
        pod setup
[✗] iOS tools - develop for iOS devices
    ✗ libimobiledevice and ideviceinstaller are not installed. To install with
      Brew, run:
        brew update
        brew install --HEAD usbmuxd
        brew link usbmuxd
        brew install --HEAD libimobiledevice
        brew install ideviceinstaller
    ✗ ios-deploy not installed. To install:
        brew install ios-deploy
[!] Android Studio (version 3.4)
    ✗ Flutter plugin not installed; this adds Flutter specific functionality.
    ✗ Dart plugin not installed; this adds Dart specific functionality.
[✓] Connected device (1 available)

! Doctor found issues in 4 categories.

2.执行$ flutter doctor --android-licenses 提示

A newer version of the Android SDK is required. To update, run:
/Users/liyan/Documents/android/sdk/tools/bin/sdkmanager --update

3.执行 /Users/liyan/Documents/android/sdk/tools/bin/sdkmanager --update 提示

Exception in thread "main" java.lang.NoClassDefFoundError: javax/xml/bind/annotation/XmlSchema
    at com.android.repository.api.SchemaModule$SchemaModuleVersion.<init>(SchemaModule.java:156)
    at com.android.repository.api.SchemaModule.<init>(SchemaModule.java:75)
    at com.android.sdklib.repository.AndroidSdkHandler.<clinit>(AndroidSdkHandler.java:81)
    at com.android.sdklib.tool.sdkmanager.SdkManagerCli.main(SdkManagerCli.java:73)
    at com.android.sdklib.tool.sdkmanager.SdkManagerCli.main(SdkManagerCli.java:48)
Caused by: java.lang.ClassNotFoundException: javax.xml.bind.annotation.XmlSchema
    at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:583)
    at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:521)
    ... 5 more

查询相关资料是因为jdk版本太高，最后重新装了一下1.8的jdk才解决问题。