Revert replacing convention with extension

It seems that was wrong

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

@@ -62,10 +62,10 @@ abstract class KotlinSourceSetProcessor<T : AbstractCompile>(
     }
 
     private fun createKotlinSourceSet(): KotlinSourceSet {
-        logger.kotlinDebug("Creating KotlinSourceSet for source set $sourceSet")
+        logger.kotlinDebug("Creating KotlinSourceSet for $sourceSet")
         val kotlinSourceSet = kotlinSourceSetProvider.create(sourceSet.name)
         kotlinSourceSet.kotlin.srcDir(project.file(sourceRootDir))
-        sourceSet.addExtension(dslExtensionName, kotlinSourceSet)
+        sourceSet.addConvention(dslExtensionName, kotlinSourceSet)
         return kotlinSourceSet
     }
 
@@ -272,7 +272,7 @@ open class KotlinAndroidPlugin(
             log.kotlinDebug("Creating KotlinSourceSet for source set $sourceSet")
             val kotlinSourceSet = kotlinSourceSetProvider.create(sourceSet.name)
             kotlinSourceSet.kotlin.srcDir(project.file(project.file("src/${sourceSet.name}/kotlin")))
-            sourceSet.addExtension(KOTLIN_DSL_NAME, kotlinSourceSet)
+            sourceSet.addConvention(KOTLIN_DSL_NAME, kotlinSourceSet)
 
             aptConfigurations.put(sourceSet.name,
                     project.createAptConfiguration(sourceSet.name, kotlinAnnotationProcessingDep))
@@ -367,7 +367,7 @@ open class KotlinAndroidPlugin(
         val logger = kotlinTask.project.logger
 
         for (provider in variantData.sourceProviders) {
-            val kotlinSourceSet = provider.findExtension(KOTLIN_DSL_NAME) as? KotlinSourceSet ?: continue
+            val kotlinSourceSet = provider.getConvention(KOTLIN_DSL_NAME) as? KotlinSourceSet ?: continue
             kotlinTask.source(kotlinSourceSet.kotlin)
         }
 

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

@@ -2,6 +2,7 @@ package org.jetbrains.kotlin.gradle.plugin
 
 import org.gradle.api.Task
 import org.gradle.api.file.FileCollection
+import org.gradle.api.internal.HasConvention
 import org.gradle.api.logging.Logger
 import org.gradle.api.plugins.ExtensionAware
 import org.gradle.api.tasks.compile.AbstractCompile
@@ -33,12 +34,15 @@ fun AbstractCompile.mapClasspath(fn: ()->FileCollection) {
     conventionMapping.map("classpath", fn)
 }
 
+internal inline fun <reified T : Any> Any.addConvention(name: String, plugin: T) {
+    (this as HasConvention).convention.plugins[name] = plugin
+}
+
 internal inline fun <reified T : Any> Any.addExtension(name: String, extension: T) =
         (this as ExtensionAware).extensions.add(name, extension)
 
-internal fun Any.findExtension(name: String): Any? =
-        (this as ExtensionAware).extensions.findByName(name)
-
+internal fun Any.getConvention(name: String): Any? =
+        (this as HasConvention).convention.plugins[name]
 
 internal fun Logger.kotlinDebug(message: String) {
     this.debug("[KOTLIN] $message")