离子构建失败：
尝试了很多解决方案，但最终遇到了相同的问题：
ionic cordova运行android
Could not resolve all files for configuration ':app:debugCompileClasspath'.
> Could not resolve com.android.support:support-annotations:27.+.
Required by:
project :app
> Could not resolve com.android.support:support-annotations:27.+.
> Failed to list versions for com.android.support:support-annotations.
> Unable to load Maven meta-data from https://maven.google.com/com/android/support/support-annotations/maven-metadata.xml.
> Could not HEAD 'https://maven.google.com/com/android/support/support-annotations/maven-metadata.xml'.
> No route to host (Host unreachable)
渐变版本：5.2.1
离子版本：4