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.

Solusi: 

Tambahkan baris berikut pada app gradle. 

compile 'com.android.support:support-annotations:27.1.1'

kemudian sync gradle.