JS: move more test to box tests

js/js.translator/testData/box/propertyAccess/propertyAssignment.kt

@@ -0,0 +1,21 @@
+// See https://youtrack.jetbrains.com/issue/KT-10785
+package foo
+
+class A(var x: Int) {
+    operator fun plusAssign(other: A) {
+        x += other.x
+    }
+}
+
+object B {
+    private var holder = A(42)
+
+    val foo: A
+        get() = holder
+}
+
+fun box(): String {
+    B.foo += A(23)
+    if (B.foo.x != 65) return "failed: ${B.foo.x}"
+    return "OK"
+}