[JS TESTS] Rewrite web demo tests using new test infrastructure

js/js.translator/testData/webDemoExamples/isCheck.kt

@@ -0,0 +1,12 @@
+// MAIN_ARGS: []
+
+fun main(args: Array<String>) {
+    println(getStringLength("aaa"))
+    println(getStringLength(1))
+}
+
+fun getStringLength(obj: Any): Int? {
+    if (obj is String)
+        return obj.length // no cast to String is needed
+    return null
+}