How to debug xtend code in IntelliJ + Gradle? - intellij-idea

Question

I have just started to learn Xtend (http://xtend-lang.org/) and try to write a small app in IntelliJ 2016 based on a Gradle project using the Xtext/Xtend 2.10.0 plugins from the standard repository. The problem is that I cannot debug my Xtend code. Breakpoints are all ignored although debugging of Java code works. Debugging Xtend code only works if I use the IntelliJ wizard and create a standard IntelliJ project without using Gradle. I don't know how to fix this? Do I miss something? Does debugging works different in a Gradle project?
Here my Gradle file:
group 'GradleXtend'
version '1.0-SNAPSHOT'
apply plugin: 'java'
apply plugin: 'org.xtext.xtend'
sourceCompatibility = 1.5
repositories {
jcenter()
mavenCentral()
}
dependencies {
compile 'org.eclipse.xtend:org.eclipse.xtend.lib:2.10.0'
testCompile group: 'junit', name: 'junit', version: '4.11'
}
buildscript {
repositories {
jcenter()
}
dependencies {
classpath 'org.xtext:xtext-gradle-plugin:1.0.12'
}
}
xtend {
debugger {
//how to install debug info into generated Java code
//SMAP adds Xtend debug info on top of Java
//PRIMARY makes Xtend the only debug info (throws away Java line numbers)
//default is SMAP for Java projects and PRIMARY for Android
sourceInstaller = 'SMAP' //or 'PRIMARY' or 'NONE'
//whether to hide synthetic variables in the debugger
hideSyntheticVariables = false
}
}
My Xtend file:
package HelloWorld
class HelloWorld2 {
def static void main(String[] args) {
println("Hello")
}
}