True. Sorry about that. These are the errors:
$ npx google-closure-compiler --js=all.js --externs all.externs.js --js_output_file=out.min.js --compilation_level ADVANCED_OPTIMIZATIONS --language_in UNSTABLE
... warnings ...
all.js:8831:11: ERROR - [JSC_UNDEFINED_VARIABLE] variable MK_INTEGER_S is undeclared
8831| return MK_INTEGER_S(parseInt(str, 10));
^^^^^^^^^^^^
all.js:8833:11: ERROR - [JSC_UNDEFINED_VARIABLE] variable h$ghcjsbn_readInteger is undeclared
8833| return h$ghcjsbn_readInteger(str);
^^^^^^^^^^^^^^^^^^^^^
all.js:8904:126: ERROR - [JSC_UNDEFINED_VARIABLE] variable a is undeclared
8904| while(--i>=0) r = (h$c2(h$ghczmprimZCGHCziTypesziZC_con_e, ((h$c1(h$ghczminternalZCGHCziInternalziJSziPrimziJSVal_con_e, (a[i])))), (r)));
^
all.js:8931:14: ERROR - [JSC_UNDEFINED_VARIABLE] variable ls is undeclared
8931| for(var i=ls-k;i>=0;i-=k) r = (h$c2(h$ghczmprimZCGHCziTypesziZC_con_e, ((h$c1(h$ghczminternalZCGHCziInternalziJSziPrimziJSVal_con_e, (x.substr(i,i+k))))), (r)));
^^
all.js:19980:0: ERROR - [JSC_UNDEFINED_VARIABLE] variable h$stg_absentErrorzh is undeclared
19980| h$stg_absentErrorzh(b,c);
^^^^^^^^^^^^^^^^^^^
all.js:19986:0: ERROR - [JSC_UNDEFINED_VARIABLE] variable h$stg_paniczh is undeclared
19986| h$stg_paniczh(h$ghczmprimZCGHCziPrimziPaniczilvl_1,h$ghczmprimZCGHCziPrimziPaniczilvl_2);
^^^^^^^^^^^^^
6 error(s), 80 warning(s)
UPDATE:
It seems like I have gotten it to work now thanks to the old wiki that @alexfmpe linked.
I used the additional option --jscomp_off=checkVars and then it completed successfully.
So my command was: $ npx google-closure-compiler --js=all.js --jscomp_off=checkVars --externs all.externs.js --js_output_file=out.min.js --compilation_level ADVANCED_OPTIMIZATIONS --language_in UNSTABLE
The file is now 3.3 MiB. And from my initial testing everything seems to still work as expected!
But I assume I should also use the following two additional options: --isolation_mode=IIFE --assume_function_wrapper as documented in the old wiki and its also used in the Makefile mentioned before.