解决方法： 看起来 onesignal 插件对 flutter_tools 有依赖性。解决方案是在“android/app/build.gradle”中按顺序将 onesignal 插件放在它后面：
\napply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"\napply plugin: 'com.onesignal.androidsdk.onesignal-gradle-plugin'\n将 Flutter 版本从 2.8.1 升级到 2.10 后，我开始收到以下错误。我重新安装了Flutter，但结果没有改变。当我将 Flutter 降级到 2.8.1 时，它可以正常工作，但是当我升级到 2.10 时，它就不行了。
\n\nRunning Gradle task 'assembleDebug'...\n\nFAILURE: Build failed with an exception.\n\n* Where:\nScript '/Users/aksoyhlc/flutter/flutter/packages/flutter_tools/gradle/flutter.gradle' line: 421\n\n What went wrong:\nA problem occurred evaluating root project 'android'.\n> A problem occurred configuring project ':app'.\n > **Cannot run Project.afterEvaluate(Closure) when the project is already evaluated.**\n\n* Try:\nRun with --stacktrace option to get the stack trace. Run with --info or …