1.修改 flutter安装目录/packages/flutter_tools/gradle/resolve_dependencies.gradle 文件

// 大约在19行
repositories {
    google()
    jcenter()
    maven {
        // 修改处
        // url "$storageUrl/download.flutter.io"
        url "http://download.flutter.io"
    }
}

2.修改 flutter/packages/flutter_tools/gradle/flutter.gradle 文件

buildscript {
    repositories {
        // 这里做了修改，使用国内阿里的代理
        // google()
        // jcenter()
        maven { url 'https://maven.aliyun.com/repository/google' }
        maven { url 'https://maven.aliyun.com/repository/jcenter' }
        maven { url 'http://maven.aliyun.com/nexus/content/groups/public' }
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:4.1.0'
    }
}

// 修改处，大约在44行
// private static final String DEFAULT_MAVEN_HOST = "https://storage.googleapis.com";
private static final String DEFAULT_MAVEN_HOST = "https://storage.flutter-io.cn/download.flutter.io";