Fix @OutputFile annotation on a String property outputFile.

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

@@ -296,8 +296,8 @@ internal class Kotlin2JsSourceSetProcessor(
             val appliedPlugins = subpluginEnvironment.addSubpluginOptions(
                     project, kotlinTask, kotlinTask, null, null, sourceSet)
 
-            kotlinTask.kotlinOptions.outputFile = File(kotlinTask.outputFile).absolutePath
-            val outputDir = File(kotlinTask.outputFile).parentFile
+            kotlinTask.kotlinOptions.outputFile = kotlinTask.outputFile.absolutePath
+            val outputDir = kotlinTask.outputFile.parentFile
 
             if (FileUtil.isAncestor(outputDir, project.rootDir, false))
                 throw InvalidUserDataException(
@@ -323,7 +323,7 @@ internal class Kotlin2JsSourceSetProcessor(
         val task = project.tasks.create(taskName, Delete::class.java)
         task.onlyIf { kotlinTask.kotlinOptions.sourceMap }
         task.delete(object : Closure<String>(this) {
-            override fun call(): String? = (kotlinTask.property("outputFile") as String) + ".map"
+            override fun call(): String? = (kotlinTask.property("outputFile") as File).canonicalPath + ".map"
         })
         project.tasks.findByName("clean")?.dependsOn(taskName)
     }

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

@@ -445,8 +445,8 @@ open class Kotlin2JsCompile() : AbstractKotlinCompile<K2JSCompilerArguments>(),
 
     @Suppress("unused")
     @get:OutputFile
-    val outputFile: String
-        get() = kotlinOptions.outputFile ?: defaultOutputFile.canonicalPath
+    val outputFile: File
+        get() = kotlinOptions.outputFile?.let(::File) ?: defaultOutputFile
 
     override fun findKotlinCompilerClasspath(project: Project): List<File> =
             findKotlinJsCompilerClasspath(project)
@@ -458,7 +458,7 @@ open class Kotlin2JsCompile() : AbstractKotlinCompile<K2JSCompilerArguments>(),
         args.apply { fillDefaultValues() }
         super.setupCompilerArgs(args, defaultsOnly)
 
-        args.outputFile = outputFile
+        args.outputFile = outputFile.canonicalPath
 
         if (defaultsOnly) return
 
@@ -472,8 +472,7 @@ open class Kotlin2JsCompile() : AbstractKotlinCompile<K2JSCompilerArguments>(),
     internal val friendDependency
         get() = friendTaskName
                 ?.let { project.getTasksByName(it, false).singleOrNull() as? Kotlin2JsCompile }
-                ?.outputFile
-                ?.let { File(it).parentFile }
+                ?.outputFile?.parentFile
                 ?.let { if (LibraryUtils.isKotlinJavascriptLibrary(it)) it else null }
                 ?.absolutePath