异常场景
从AS2.0升级到2.1，重新编译工程后，抛出了如下异常
Error:A problem occurred configuring root project 'AndroidStudioProjects'. > Could not resolve all dependencies for configuration ':classpath'. > Could not resolve com.android.tools.build:gradle:2.0.0. Required by: :AndroidStudioProjects:unspecified > No cached version of com.android.tools.build:gradle:2.0.0 available for offline mode.
分析
之前为了优化gradle的编译速度，选择了Offline Work模式，取消即可
解决
Settings-Build.Execution,Deployment-Build Tools-Gradle–取消勾选的Offline Work, 重新编译，AS会自动下载.
