Set scratch file extension using API for IDEA 182 or higher

idea/idea-jvm/src/org/jetbrains/kotlin/idea/scratch/KtScratchFileCreationHelper.kt

@@ -0,0 +1,20 @@
+/*
+ * 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.idea.scratch
+
+import com.intellij.ide.scratch.ScratchFileCreationHelper
+import com.intellij.openapi.actionSystem.DataContext
+import com.intellij.openapi.project.Project
+import org.jetbrains.kotlin.parsing.KotlinParserDefinition
+
+class KtScratchFileCreationHelper: ScratchFileCreationHelper() {
+
+    override fun prepareText(project: Project, context: Context, dataContext: DataContext): Boolean {
+        context.fileExtension = KotlinParserDefinition.STD_SCRIPT_SUFFIX
+
+        return super.prepareText(project, context, dataContext)
+    }
+}

idea/idea-jvm/src/org/jetbrains/kotlin/idea/scratch/ScratchFileModuleInfoProvider.kt

@@ -48,7 +48,7 @@ class ScratchFileModuleInfoProvider(val project: Project) : ProjectComponent {
 
             val ktFile = PsiManager.getInstance(project).findFile(file) as? KtFile ?: return
 
-            // Hack before api in IDEA will be introduced
+            // BUNCH: 181 scratch files are created with .kt extension
             if (file.extension == KotlinFileType.EXTENSION) {
                 runWriteAction {
                     var newName = file.nameWithoutExtension + STD_SCRIPT_EXT

idea/resources/META-INF/jvm.xml

@@ -122,6 +122,9 @@
 
         <editorNotificationProvider implementation="org.jetbrains.kotlin.idea.versions.UnsupportedAbiVersionNotificationPanelProvider"/>
 
+        <scratch.creationHelper language="kotlin"
+                                implementationClass="org.jetbrains.kotlin.idea.scratch.KtScratchFileCreationHelper"/>
+
         <localInspection
             groupName="Plugin DevKit"
             shortName="IncompatibleAPI"