Looking to solve your problem in 2-toplevel.tar.gz code, I've proceeded to
notice that out of all the js_of_ocaml libraries installed here, there is
missing js_of_ocaml-toplevel -- here is what happened when I try to
download and install it with OPAM (2.0.4~):

~/Downloads/p/toplevel $ opam install js_of_ocaml-toplevel
The following actions will be performed:
  ∗ install js_of_ocaml-toplevel 3.4.0

<><> Gathering sources
><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
[js_of_ocaml-toplevel.3.4.0] found in cache

<><> Processing actions
<><><><><><><><><><><><><><><><><><><><><><><><><><><><>
[ERROR] The compilation of js_of_ocaml-toplevel failed at
"/home/user/.opam/opam-init/hooks/sandbox.sh build dune build -p
js_of_ocaml-toplevel -j
        1".

#=== ERROR while compiling js_of_ocaml-toplevel.3.4.0
=========================#
# context              2.0.4 | linux/x86_64 | ocaml-base-compiler.4.07.1 |
https://opam.ocaml.org#a7ba7031
# path
~/.opam/default/.opam-switch/build/js_of_ocaml-toplevel.3.4.0
# command              ~/.opam/opam-init/hooks/sandbox.sh build dune build
-p js_of_ocaml-toplevel -j 1
# exit-code            1
# env-file             ~/.opam/log/js_of_ocaml-toplevel-3670-05296d.env
# output-file          ~/.opam/log/js_of_ocaml-toplevel-3670-05296d.out
### output ###
# Error: Unbound module Js_of_ocaml_compiler.Stdlib
# [...]
# (cd _build/default && /home/user/.opam/default/bin/ocamlc.opt -w -40 -g
-bin-annot -I toplevel/lib/.js_of_ocaml_toplevel.objs/byte -I
/home/user/.opam/default/lib/biniou -I /home/user/.opam/default/lib/bytes
-I /home/user/.opam/default/lib/easy-format -I
/home/user/.opam/default/lib/js_of_ocaml -I
/home/user/.opam/default/lib/js_of_ocaml-compiler -I
/home/user/.opam/default/lib/[...]
# File "toplevel/lib/jsooTopPpx.ml", line 20, characters 5-32:
# Error: Unbound module Js_of_ocaml_compiler.Stdlib
#       ocamlc
toplevel/lib/.js_of_ocaml_toplevel.objs/byte/js_of_ocaml_toplevel__JsooTop.{cmo,cmt}
(exit 2)
# (cd _build/default && /home/user/.opam/default/bin/ocamlc.opt -w -40 -g
-bin-annot -I toplevel/lib/.js_of_ocaml_toplevel.objs/byte -I
/home/user/.opam/default/lib/biniou -I /home/user/.opam/default/lib/bytes
-I /home/user/.opam/default/lib/easy-format -I
/home/user/.opam/default/lib/js_of_ocaml -I
/home/user/.opam/default/lib/js_of_ocaml-compiler -I
/home/user/.opam/default/lib/[...]
# File "toplevel/lib/jsooTop.ml", line 22, characters 5-32:
# Error: Unbound module Js_of_ocaml_compiler.Stdlib
#     ocamlopt
toplevel/lib/.js_of_ocaml_toplevel.objs/native/js_of_ocaml_toplevel__JsooTopPpx.{cmx,o}
(exit 2)
# (cd _build/default && /home/user/.opam/default/bin/ocamlopt.opt -w -40 -g
-I toplevel/lib/.js_of_ocaml_toplevel.objs/byte -I
toplevel/lib/.js_of_ocaml_toplevel.objs/native -I
/home/user/.opam/default/lib/biniou -I /home/user/.opam/default/lib/bytes
-I /home/user/.opam/default/lib/easy-format -I
/home/user/.opam/default/lib/js_of_ocaml -I
/home/user/.opam/default/lib/js_of_ocaml-co[...]
# File "toplevel/lib/jsooTopPpx.ml", line 20, characters 5-32:
# Error: Unbound module Js_of_ocaml_compiler.Stdlib



<><> Error report
<><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
┌─ The following actions failed
│ λ build js_of_ocaml-toplevel 3.4.0
└─
╶─ No changes have been performed
~/Downloads/p/toplevel $ opam install js_of_ocaml-compiler
[NOTE] Package js_of_ocaml-compiler is already installed (current version
is 3.3.0).