KJS: introduce new base class WebDemoExamples* tests

js/js.translator/testData/webDemoExamples1/nullChecks.kt

@@ -0,0 +1,19 @@
+// Return null if str does not hold a number
+fun myParseInt(str: String): Int? = str.toIntOrNull().also {
+    if (it == null) println("One of arguments isn't Int")
+}
+
+fun main(args: Array<String>) {
+    if (args.size < 2) {
+        print("No number supplied");
+    }
+    else {
+        val x = myParseInt(args[0])
+        val y = myParseInt(args[1])
+
+        // We cannot say 'x * y' now because they may hold nulls
+        if (x != null && y != null) {
+            print(x * y) // Now we can
+        }
+    }
+}