JS: fix translation of augmented assignment in class initializer. See KT-15569

compiler/testData/codegen/box/operatorConventions/augmentedAssignmentInInitializer.kt

@@ -0,0 +1,45 @@
+abstract class A {
+    val b = B("O")
+
+    open val c: B
+
+    val d: B
+        get() = field
+
+    var e: String
+        get() = field
+
+    init {
+        c = B("O")
+        d = B("O")
+        e = "O"
+
+        b += ","
+        c += "."
+        d += ";"
+        e += "|"
+    }
+}
+
+class B(var value: String) {
+    operator fun plusAssign(o: String) {
+        value += o
+    }
+}
+
+class C : A() {
+    init {
+        b += "K"
+        c += "K"
+        d += "K"
+        e += "K"
+    }
+}
+
+fun box(): String {
+    val c = C()
+    val result = "${c.b.value} ${c.c.value} ${c.d.value} ${c.e}"
+    if (result != "O,K O.K O;K O|K") return "fail: $result"
+
+    return "OK"
+}