JS: move more test to box tests

js/js.translator/testData/box/operatorOverloading/overloadPlusAssignViaExtensionFunction.kt

@@ -0,0 +1,27 @@
+package foo
+
+import java.util.*
+
+class A<T>(val list: MutableList<T>) {
+    fun addAll(c: Collection<T>) {
+        list.addAll(c)
+    }
+}
+
+operator fun <T> A<T>.plusAssign(other: Collection<T>) {
+    addAll(other)
+}
+
+fun box(): String {
+    var v1 = arrayListOf("foo")
+    val v2 = listOf("bar")
+
+    val a = A(v1)
+    a += v2
+
+    if (v1.size != 2) return "fail1: ${v1.size}"
+    if (v1[0] != "foo") return "fail2: ${v1[0]}"
+    if (v1[1] != "bar") return "fail3: ${v1[1]}"
+
+    return "OK"
+}