转载自：http://stackoverflow.com/questions/40160968/android-studio-error-failed-to-resolve-com-android-support-constraintconstr
问题：
Android Studio -Error: Failed to resolve: com.android.support.constraint:constraint-layout:1.0.0-alpha1
解决方法：
You need add check in..
open Settings -> Appearance & Behavior -> System Settings -> Android SDK -> SDK Tools
And in this dialog add:
ConstraintLayout for Android
Solver for ConstrainLayout
qIjf6.png
Then, here only you need close and open Android Studio.
Maybe only you can update to:
compile 'com.android.support.constraint:constraint-layout:1.0.0-alpha8'
已测试此方法有效。
