Android Studio报错：ERROR: Failed to resolve: com.android.support:appcompat-v7:28.0.+

ERROR: Failed to resolve: com.android.support:appcompat-v7:28.0.+
Add Google Maven repository and sync project
Show in Project Structure dialog
Affected Modules: app

ERROR: Failed to resolve: com.android.support:recyclerview-v7:28.0.0
Add Google Maven repository and sync project
Show in Project Structure dialog
Affected Modules: app

ERROR: Failed to resolve: com.android.support.test.espresso:espresso-core:2.2.2
Add Google Maven repository and sync project
Show in Project Structure dialog
Affected Modules: app

尝试各种Clear、Rebuild无果后放弃了，还得去StackOverflow寻找良方，果然不负众望，找到了答案：
Failed to resolve: com.android.support:appcompat-v7:28.0
项目级Gradle中repositories的jcenter()下一行添加如下maven代码即可：

allprojects {
    repositories {
        jcenter()
        maven {// here we go!!
            url 'https://maven.google.com'
        }
    }
}

再编译，完美运行。

原来 Android Support Library dependencies 都是从 Android SDK Manager 里面下载的，现在新版本必须从 Google 的 Maven 仓库里面去下载。