Implement language settings API for Kotlin source sets

libraries/tools/kotlin-gradle-plugin-api/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/LanguageSettingsBuilder.kt

@@ -0,0 +1,18 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o. 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.gradle.plugin
+
+interface LanguageSettingsBuilder {
+    var languageVersion: String?
+
+    var apiVersion: String?
+
+    var progressiveMode: Boolean
+
+    fun enableLanguageFeature(name: String)
+
+    val enabledLanguageFeatures: Set<String>
+}

libraries/tools/kotlin-gradle-plugin-api/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/source/KotlinSourceSet.kt

@@ -9,19 +9,22 @@ import groovy.lang.Closure
 import org.gradle.api.Named
 import org.gradle.api.file.SourceDirectorySet
 import org.jetbrains.kotlin.gradle.plugin.HasKotlinDependencies
+import org.jetbrains.kotlin.gradle.plugin.LanguageSettingsBuilder
 
 interface KotlinSourceSet : Named, HasKotlinDependencies {
     val kotlin: SourceDirectorySet
+    fun kotlin(configureClosure: Closure<Any?>): SourceDirectorySet
+
     val resources: SourceDirectorySet
 
-    fun kotlin(configureClosure: Closure<Any?>): SourceDirectorySet
+    val languageSettings: LanguageSettingsBuilder
+    fun languageSettings(configureClosure: Closure<Any?>): LanguageSettingsBuilder
+
+    fun dependsOn(other: KotlinSourceSet)
+    val dependsOn: Set<KotlinSourceSet>
 
     companion object {
         const val COMMON_MAIN_SOURCE_SET_NAME = "commonMain"
         const val COMMON_TEST_SOURCE_SET_NAME = "commonTest"
     }
-
-    fun dependsOn(other: KotlinSourceSet)
-
-    val dependsOn: Set<KotlinSourceSet>
 }