JS backend: fixed wrong NPE when casting to generic type.

js/js.translator/testData/expression/cast/cases/castToGenericType.kt

@@ -0,0 +1,74 @@
+package foo
+
+class A(val s: String)
+
+fun castsNotNullToNullableT<T>(a: Any) {
+    a as T
+    a as T?
+    a as? T
+    a as? T?
+}
+
+fun castsNullableToNullableT<T>(a: Any?) {
+    a as T
+    a as T?
+    a as? T
+    a as? T?
+}
+
+
+fun castsNotNullToNotNullT<T : Any>(a: Any) {
+    a as T
+    a as T?
+    a as? T
+    a as? T?
+}
+
+fun castNullableToNotNullT<T : Any>(a: Any?) {
+    a as T
+}
+
+fun castsNullableToNotNullT<T : Any>(a: Any?) {
+    a as T?
+    a as? T
+    a as? T?
+}
+
+fun test(f: () -> Unit) {
+    try {
+        f()
+    }
+    catch(e: Exception) {
+        throw Exception("Failed in $f with unexpected exception $e")
+    }
+}
+
+fun fails(f: () -> Unit) {
+    try {
+        f()
+    }
+    catch(e: Exception) {
+        return
+    }
+
+    throw Exception("Expected an exception to be thrown from $f")
+}
+
+fun box(): String {
+    val a = A("OK")
+
+    test { castsNotNullToNullableT<A>(a) }
+
+    test { castsNullableToNullableT<A>(a) }
+    test { castsNullableToNullableT<A>(null) }
+
+    test { castsNotNullToNotNullT<A>(a) }
+
+    test { castsNullableToNotNullT<A>(a) }
+    test { castsNullableToNotNullT<A>(null) }
+
+    test  { castNullableToNotNullT<A>(a) }
+    fails { castNullableToNotNullT<A>(null) }
+
+    return "OK"
+}