Builtins: made Unit object.

core/builtins/src/kotlin/Unit.kt

@@ -16,10 +16,16 @@
 
 package kotlin
 
-public class Unit private() {
-    override fun toString() = "Unit.VALUE"
-
+// Temporary class for backward compatibility with Java code
+private open class OldUnit() {
     class object {
-        public val VALUE: Unit = Unit()
+        public val VALUE: Unit = Unit
     }
 }
+
+public object Unit : OldUnit() {
+    public val VALUE: Unit
+        get() = this
+
+    override fun toString() = "Unit.VALUE"
+}