add test files

js/js.translator/testFiles/operatorOverloading/cases/plusAssignNoReassign.kt

@@ -0,0 +1,17 @@
+package foo
+
+class A() {
+
+    var message = ""
+    fun plusAssign(other : A) {
+        message = message + "!"
+    }
+
+}
+
+fun box() : Boolean {
+    var c = A()
+    c += A()
+    c += A()
+    return c.message == "!!"
+}