JS backend: fix translation when statement -- now WhenTranslator returns JsBlock instead of hackish adding statements to context block.

It fixed the compiler crashing when translating `for`/`when`/`if` with `when` statement(KT-5234) and KT-5058.

 #KT-5234 fixed
 #EA-57346 fixed
 #KT-5058 fixed

js/js.translator/testData/expression/misc/cases/KT-5058.kt

@@ -0,0 +1,25 @@
+package foo
+
+fun test():Any {
+    val a: Any = "OK"
+    val f: Any =
+            if (true) {
+                when {
+                    false -> "1"
+                    ((a as? String)?.size ?: 0 > 0) -> a
+                    else -> "2"
+                }
+            }
+            else {
+                "3"
+
+            }
+
+    return f
+}
+
+fun box(): Boolean {
+    var result = test()
+    return result == "OK";
+}
+