Use default compileJava9Java task to build java9 source set

libraries/stdlib/jdk7/build.gradle

@@ -28,16 +28,14 @@ sourceSets {
     }
     java9 {
         java {
-            srcDirs = ['java9']
+            srcDir 'java9'
         }
     }
 }
 
-createCompileJava9Task(project, "compileJava9", "kotlin.stdlib.jdk7")
-
 jar {
     manifestAttributes(manifest, project, 'Main', true)
-    from compileJava9.outputs
+    from sourceSets.java9.output
 }
 
 artifacts {
@@ -68,6 +66,8 @@ compileTestKotlin {
     kotlinOptions.freeCompilerArgs = ["-Xallow-kotlin-package"]
 }
 
+compileJava9Sources(project, 'kotlin.stdlib.jdk7')
+
 kotlin.experimental.coroutines 'enable'
 
 test {