js: Convert _.flatten(a, true) to [].concat(...a).
js: Convert _.flatten(a, true) to [].concat(...a).

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulipchat.com>

timabbott/zulip,rht/zulip,shubhamdhama/zulip,kou/zulip,andersk/zulip,shubhamdhama/zulip,showell/zulip,hackerkid/zulip,showell/zulip,synicalsyntax/zulip,brainwane/zulip,punchagan/zulip,brainwane/zulip,timabbott/zulip,shubhamdhama/zulip,andersk/zulip,shubhamdhama/zulip,hackerkid/zulip,zulip/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,shubhamdhama/zulip,zulip/zulip,brainwane/zulip,rht/zulip,showell/zulip,brainwane/zulip,kou/zulip,synicalsyntax/zulip,eeshangarg/zulip,punchagan/zulip,punchagan/zulip,eeshangarg/zulip,rht/zulip,synicalsyntax/zulip,kou/zulip,timabbott/zulip,brainwane/zulip,hackerkid/zulip,andersk/zulip,hackerkid/zulip,punchagan/zulip,showell/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,brainwane/zulip,eeshangarg/zulip,eeshangarg/zulip,hackerkid/zulip,hackerkid/zulip,rht/zulip,synicalsyntax/zulip,synicalsyntax/zulip,zulip/zulip,eeshangarg/zulip,brainwane/zulip,punchagan/zulip,timabbott/zulip,rht/zulip,andersk/zulip,zulip/zulip,synicalsyntax/zulip,zulip/zulip,punchagan/zulip,showell/zulip,shubhamdhama/zulip,synicalsyntax/zulip,kou/zulip,punchagan/zulip,andersk/zulip,showell/zulip,rht/zulip,andersk/zulip,kou/zulip,timabbott/zulip,showell/zulip,timabbott/zulip,kou/zulip,shubhamdhama/zulip,timabbott/zulip