Don't fail when using Unit class object as a value

#KT-4243 Fixed

compiler/testData/codegen/box/unit/unitClassObject.kt

@@ -0,0 +1,13 @@
+fun box(): String {
+    Unit
+
+    val a = Unit
+    val b = Unit
+    if (a != b) return "Fail a != b"
+
+    if (Unit != Unit) return "Fail Unit != Unit"
+
+    if (a.VALUE != Unit.VALUE) return "Fail a.VALUE != Unit.VALUE"
+
+    return "OK"
+}

compiler/tests/org/jetbrains/jet/codegen/generated/BlackBoxCodegenTestGenerated.java

@@ -4565,6 +4565,11 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             JetTestUtils.assertAllTestsPresentByMetadata(this.getClass(), "org.jetbrains.jet.generators.tests.GenerateTests", new File("compiler/testData/codegen/box/unit"), Pattern.compile("^(.+)\\.kt$"), true);
         }
         
+        @TestMetadata("unitClassObject.kt")
+        public void testUnitClassObject() throws Exception {
+            doTest("compiler/testData/codegen/box/unit/unitClassObject.kt");
+        }
+        
         @TestMetadata("UnitValue.kt")
         public void testUnitValue() throws Exception {
             doTest("compiler/testData/codegen/box/unit/UnitValue.kt");

runtime/src/jet/Unit.java

@@ -22,6 +22,8 @@ import org.jetbrains.jet.rt.annotation.AssertInvisibleInResolver;
 public class Unit {
     public static final Unit VALUE = new Unit();
 
+    public static final object object$ = new object();
+
     private Unit() {
     }
 
@@ -39,4 +41,9 @@ public class Unit {
     public int hashCode() {
         return 239;
     }
+
+    public static class object {
+        private object() {
+        }
+    }
 }