Concurrent map write in serveCommandFileSystem.Open

fatal error: concurrent map writes

goroutine 18 [running]:
runtime.throw(0x15a5217, 0x15)
        /usr/local/Cellar/go/1.11.1/libexec/src/runtime/panic.go:608 +0x72 fp=0xc003721838 sp=0xc003721808 pc=0x102c112
runtime.mapassign_faststr(0x1503240, 0xc000168ae0, 0xc0008997c8, 0x8, 0x4)
        /usr/local/Cellar/go/1.11.1/libexec/src/runtime/map_faststr.go:275 +0x3bf fp=0xc0037218a0 sp=0xc003721838 pc=0x1013d9f
main.serveCommandFileSystem.Open.func1(0xc000168e40, 0xc0001a8230, 0xc003721af0, 0x159d185, 0x1, 0x0, 0x0, 0xc00015c200, 0xc00015c4c0, 0x3, ...)
        /Users/mlnc/pkg/mod/github.com/gopherjs/gopherjs@v0.0.0-20181103185306-d547d1d9531e/tool.go:610 +0x3aa fp=0xc003721960 sp=0xc0037218a0 pc=0x149458a
main.serveCommandFileSystem.Open(0x0, 0x0, 0xc00015c200, 0xc00015c4c0, 0x3, 0x4, 0xc000168ae0, 0xc000196104, 0x5, 0x102b701, ...)
        /Users/mlnc/pkg/mod/github.com/gopherjs/gopherjs@v0.0.0-20181103185306-d547d1d9531e/tool.go:613 +0x9d4 fp=0xc003721b78 sp=0xc003721960 pc=0x148d5f4
main.(*serveCommandFileSystem).Open(0xc00015c500, 0xc000196104, 0x5, 0x1099588, 0xc000182380, 0x0, 0x0)
        <autogenerated>:1 +0x79 fp=0xc003721bf0 sp=0xc003721b78 pc=0x14952a9
net/http.serveFile(0x166e720, 0xc0002ca000, 0xc000192400, 0x16694a0, 0xc00015c500, 0xc000196104, 0x5, 0xc000168d01)
        /usr/local/Cellar/go/1.11.1/libexec/src/net/http/fs.go:551 +0xad fp=0xc003721ce8 sp=0xc003721bf0 pc=0x131d12d
net/http.(*fileHandler).ServeHTTP(0xc00016a310, 0x166e720, 0xc0002ca000, 0xc000192400)
        /usr/local/Cellar/go/1.11.1/libexec/src/net/http/fs.go:723 +0xa1 fp=0xc003721d48 sp=0xc003721ce8 pc=0x131de21
net/http.serverHandler.ServeHTTP(0xc000199110, 0x166e720, 0xc0002ca000, 0xc000192400)
        /usr/local/Cellar/go/1.11.1/libexec/src/net/http/server.go:2741 +0xab fp=0xc003721d78 sp=0xc003721d48 pc=0x135cf5b
net/http.(*conn).serve(0xc00019cb40, 0x166efa0, 0xc00015c600)
        /usr/local/Cellar/go/1.11.1/libexec/src/net/http/server.go:1847 +0x646 fp=0xc003721fc8 sp=0xc003721d78 pc=0x1359326
runtime.goexit()
        /usr/local/Cellar/go/1.11.1/libexec/src/runtime/asm_amd64.s:1333 +0x1 fp=0xc003721fd0 sp=0xc003721fc8 pc=0x1058af1
created by net/http.(*Server).Serve
        /usr/local/Cellar/go/1.11.1/libexec/src/net/http/server.go:2851 +0x2f5

Start in tool.go around lines 610-613, where the stack trace identifies serveCommandFileSystem.Open and the concurrent map write. Reproduce concurrent file-serving requests and inspect the map access there; done means concurrent opens no longer trigger the reported fatal error.