[js] Fix a bug in nqp::bindpos, make it work for negative indexes.

Author: pmurias

src/vm/js/QAST/Compiler.nqp

@@ -2990,7 +2990,7 @@ class QAST::CompilerJS does DWIMYNameMangling does SerializeOnce {
 
     method bind_pos($array, $index, $value, :$node) {
         my $array_chunk := self.as_js($array, :want($T_OBJ));
-        my $index_chunk := self.as_js($index, :want($T_STR));
+        my $index_chunk := self.as_js($index, :want($T_INT));
         my $value_chunk := self.as_js($value, :want($T_OBJ));
 
         Chunk.new($T_OBJ, $value_chunk.expr, [$array_chunk, $index_chunk, $value_chunk, "nqp.op.bindpos({$array_chunk.expr},{$index_chunk.expr},{$value_chunk.expr});\n"], :node($node));

src/vm/js/nqp-runtime/core.js

@@ -23,6 +23,9 @@ op.atpos = function(array, index) {
 
 op.bindpos = function(array, index, value) {
   if (array instanceof Array) {
+    if (index < 0) {
+      index = array.length + index;
+    }
     return (array[index] = value);
   } else {
     return array.$$bindpos(index, value);