Provide default output JS file in Kotlin Gradle plugin

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/Kotlin2JsGradlePluginIT.kt

@@ -66,12 +66,12 @@ class Kotlin2JsGradlePluginIT : BaseGradleIT() {
     }
 
     @Test
-    fun testNoOutputFileFails() {
+    fun testDefaultOutputFile() {
         val project = Project("kotlin2JsNoOutputFileProject", "2.10")
+
         project.build("build") {
-            assertFailed()
-            assertReportExists()
-            assertContains("compileKotlin2Js.kotlinOptions.outputFile should be specified.")
+            assertSuccessful()
+            assertFileExists("build/classes/main/kotlin2JsNoOutputFileProject_main.js")
         }
     }
 

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

@@ -55,7 +55,8 @@ internal abstract class KotlinSourceSetProcessor<T : AbstractKotlinCompile<*>>(
     protected val sourceRootDir: String = "src/$sourceSetName/kotlin"
     protected val kotlinSourceSet: KotlinSourceSet = createKotlinSourceSet()
     protected val kotlinTask: T = createKotlinCompileTask()
-    protected abstract val defaultKotlinDestinationDir: File
+    protected open val defaultKotlinDestinationDir: File
+            get() = sourceSet.output.classesDir
 
     fun run() {
         addKotlinDirSetToSources()
@@ -183,9 +184,6 @@ internal class Kotlin2JsSourceSetProcessor(
         taskDescription = "Compiles the kotlin sources in $sourceSet to JavaScript.",
         compileTaskNameSuffix = "kotlin2Js"
 ) {
-    override val defaultKotlinDestinationDir: File
-        get() = File(project.buildDir, "kotlin2js/$sourceSetName")
-
     private val clean = project.tasks.findByName("clean")
     private val build = project.tasks.findByName("build")
 

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

@@ -20,6 +20,7 @@ import org.codehaus.groovy.runtime.MethodClosure
 import org.gradle.api.GradleException
 import org.gradle.api.Task
 import org.gradle.api.logging.Logger
+import org.gradle.api.tasks.SourceSet
 import org.gradle.api.tasks.SourceTask
 import org.gradle.api.tasks.TaskAction
 import org.gradle.api.tasks.compile.AbstractCompile
@@ -299,6 +300,9 @@ open class Kotlin2JsCompile() : AbstractKotlinCompile<K2JSCompilerArguments>(),
 
     var compilerJarFile: File? = null
 
+    private val defaultOutputFile: File
+            get() = File(destinationDir, "$moduleName.js")
+
     @Suppress("unused")
     val outputFile: String?
         get() = kotlinOptions.outputFile
@@ -340,7 +344,7 @@ open class Kotlin2JsCompile() : AbstractKotlinCompile<K2JSCompilerArguments>(),
         args.freeArgs = args.freeArgs + sourceRoots.kotlinSourceFiles.map { it.absolutePath }
 
         if (args.outputFile == null) {
-            throw GradleException("$name.kotlinOptions.outputFile should be specified.")
+            args.outputFile = defaultOutputFile.canonicalPath
         }
 
         logger.kotlinDebug("compiling with args ${ArgumentUtils.convertArgumentsToStringList(args)}")

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

@@ -33,6 +33,7 @@ internal open class KotlinTasksProvider {
 
     fun createKotlinJSTask(project: Project, name: String, sourceSetName: String): Kotlin2JsCompile =
             project.tasks.create(name, Kotlin2JsCompile::class.java).apply {
+                this.sourceSetName = sourceSetName
                 friendTaskName = taskToFriendTaskMapper[this]
             }