Add compiler argument to support opt-in annotations

build.gradle

@@ -28,6 +28,11 @@ allprojects {
         options.addStringOption('Xdoclint:none', '-quiet')
         options.addStringOption('encoding', 'UTF-8')
     }
+    tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).configureEach {
+        kotlinOptions {
+            freeCompilerArgs += ['-opt-in=kotlin.RequiresOptIn']
+        }
+    }
 }
 
 ext {