问题
Error:Execution failed for task ':app:preDebugAndroidTestBuild'.
> Conflict with dependency 'com.android.support:support-annotations' in project ':app'. Resolved versions for app (26.1.0) and test app (27.1.1) differ. See https://d.android.com/r/tools/test-apk-dependency-conflicts.html for details.
发生这类型的错误，是当我们修改了.build中的compileSdkVersion，产生所依赖的dependency与当前版本不一致导致的。
解决
build->Rebuid-project