Explicit Api mode: prototype with check for missing explicit visibility

Add cli flag with 3-state switch

compiler/util/src/org/jetbrains/kotlin/config/AnalysisFlag.kt

@@ -38,6 +38,10 @@ class AnalysisFlag<out T> internal constructor(
             operator fun provideDelegate(instance: Any?, property: KProperty<*>) = Delegate(property.name, JvmDefaultMode.DISABLE)
         }
 
+        object ApiModeDisabledByDefault {
+            operator fun provideDelegate(instance: Any?, property: KProperty<*>) = Delegate(property.name, ApiMode.DISABLED)
+        }
+
         object ListOfStrings {
             operator fun provideDelegate(instance: Any?, property: KProperty<*>) = Delegate(property.name, emptyList<String>())
         }

compiler/util/src/org/jetbrains/kotlin/config/ApiMode.kt

@@ -0,0 +1,18 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.config
+
+enum class ApiMode(val state: String) {
+    DISABLED("disable"),
+    ENABLED("enable"),
+    MIGRATION("migration");
+
+    companion object {
+        fun fromString(string: String): ApiMode? = values().find { it.state == string }
+
+        fun availableValues() = values().joinToString(prefix = "{", postfix = "}") { it.state }
+    }
+}