js-on-sx: Array.prototype flat + fill; fix indexOf start arg

flat: walk with depth, recursive when element is list and depth>0.
fill(value, start?, end?): in-place mutation, returns self.
indexOf: honor second arg as start position.

396/398 unit (+5), 148/148 slice unchanged.

lib/js/runtime.sx

@@ -775,7 +775,13 @@
       ((= name "indexOf")
         (fn
           (&rest args)
-          (if (= (len args) 0) -1 (js-list-index-of arr (nth args 0) 0))))
+          (if
+            (= (len args) 0)
+            -1
+            (js-list-index-of
+              arr
+              (nth args 0)
+              (if (< (len args) 2) 0 (js-num-to-int (nth args 1)))))))
       ((= name "join")
         (fn
           (&rest args)
@@ -810,6 +816,25 @@
       ((= name "every") (fn (f) (js-list-every-loop f arr 0)))
       ((= name "reverse")
         (fn () (js-list-reverse-loop arr (- (len arr) 1) (list))))
+      ((= name "flat")
+        (fn
+          (&rest args)
+          (let
+            ((depth (if (= (len args) 0) 1 (js-num-to-int (nth args 0)))))
+            (js-list-flat-loop arr depth (list)))))
+      ((= name "fill")
+        (fn
+          (&rest args)
+          (let
+            ((v (if (= (len args) 0) js-undefined (nth args 0)))
+              (s (if (< (len args) 2) 0 (js-num-to-int (nth args 1))))
+              (e
+                (if
+                  (< (len args) 3)
+                  (len arr)
+                  (js-num-to-int (nth args 2)))))
+            (js-list-fill-loop arr v s e)
+            arr)))
       (else js-undefined))))
 
 (define pop-last! (fn (lst) nil))
@@ -960,6 +985,30 @@
       ((js-to-boolean (f (nth arr i))) true)
       (else (js-list-some-loop f arr (+ i 1))))))
 
+(define
+  js-list-flat-loop
+  (fn
+    (arr depth acc)
+    (for-each
+      (fn
+        (x)
+        (if
+          (and (list? x) (> depth 0))
+          (js-list-flat-loop x (- depth 1) acc)
+          (append! acc x)))
+      arr)
+    acc))
+
+(define
+  js-list-fill-loop
+  (fn
+    (arr v s e)
+    (cond
+      ((>= s e) nil)
+      ((>= s (len arr)) nil)
+      (else
+        (begin (js-list-set! arr s v) (js-list-fill-loop arr v (+ s 1) e))))))
+
 (define
   js-list-every-loop
   (fn
@@ -1219,6 +1268,8 @@
             ((= key "some") (js-array-method obj "some"))
             ((= key "every") (js-array-method obj "every"))
             ((= key "reverse") (js-array-method obj "reverse"))
+            ((= key "flat") (js-array-method obj "flat"))
+            ((= key "fill") (js-array-method obj "fill"))
             (else js-undefined)))
         ((= (type-of obj) "string")
           (cond