Add tests for separate classes dirs

(cherry picked from commit b07d79b)

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

@@ -225,4 +225,14 @@ class Kotlin2JsGradlePluginIT : BaseGradleIT() {
             assertSuccessful()
         }
     }
+
+    /** Issue: KT-18495 */
+    @Test
+    fun testNoSeparateClassesDirWarning() {
+        val project = Project("kotlin2JsProject", "4.0")
+        project.build("build") {
+            assertSuccessful()
+            assertNotContains("this build assumes a single directory for all classes from a source set")
+        }
+    }
 }

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

@@ -24,6 +24,8 @@ import org.jetbrains.kotlin.gradle.util.getFilesByNames
 import org.jetbrains.kotlin.gradle.util.modify
 import org.junit.Test
 import java.io.File
+import java.util.zip.ZipFile
+import kotlin.test.assertEquals
 import kotlin.test.assertNotEquals
 import kotlin.test.assertTrue
 
@@ -542,4 +544,27 @@ class KotlinGradleIT: BaseGradleIT() {
             assertContains("-api-version 1.1")
         }
     }
+
+    @Test
+    fun testSeparateOutputGradle40() {
+        val project = Project("kotlinJavaProject", "4.0")
+        project.build("compileDeployKotlin", "assemble") {
+            assertSuccessful()
+
+            // Check that the Kotlin classes are placed under directories following the guideline:
+            assertFileExists("build/classes/kotlin/main/demo/KotlinGreetingJoiner.class")
+            assertFileExists("build/classes/kotlin/deploy/demo/ExampleSource.class")
+
+            // Check that the resulting JAR contains the Kotlin classes, without duplicates:
+            val jar = ZipFile(fileInWorkingDir("build/libs/${project.projectName}.jar"))
+            assertEquals(1, jar.entries().asSequence().count { it.name == "demo/KotlinGreetingJoiner.class" })
+
+            // Check that the Java output is intact:
+            assertFileExists("build/classes/java/main/demo/Greeter.class")
+
+            // Check that the sync output task is not used with Gradle 4.0+ and there's no old Kotlin output layout
+            assertNotContains(":copyMainKotlinClasses")
+            assertNoSuchFile("build/kotlin-classes")
+        }
+    }
 }