Web.SocketIO fails with fatal "Internal error: Not allowed to add more identifiers during second compiler pass."

Imported from https://youtrack.roxen.com/issue/PIKE-31
Reported by @grubba
/home/hungerf3/xeno/xenoclient/[pike-8](#8008).1/cruncher.ofdoom.com/buildtmp/Pike-v8.1-snapshot/src/program.c:6791: Fatal error:
Internal error: Not allowed to add more identifiers during second compiler pass.
Added identifier: "register"
Backtrace at time of fatal:
-:1: PikeCompiler("", UNDEFINED, -1, -1, target, placeholder)->compile()
-:1: DefaultCompilerEnvironment->compile(PikeCompiler("", UNDEFINED, -1, -1, target, placeholder))
test-install/pike/8.1.11/lib/master.pike:670: compile_string("/*\n\40*\40Clean-room\40Socket.IO\40implementation\40for\40Pike.\n\40*/\n\n//!\40This\40is\40a"+[22574]+"\40\40\40return\40res;\n\40\40}\n}\n","/home/hungerf3/xeno/xenoclient/[pike-8](#8008).1/cruncher.ofdoom.com/buildtmp/P"+[90]+"eb.pmod/SocketIO.pmod",0,,,2)
test-install/pike/8.1.11/lib/master.pike:1476: master()->low_findprog("/home/hungerf3/xeno/xenoclient/[pike-8](#8008).1/cruncher.ofdoom.com/buildtmp/Pike-v8.1-snapshot/build/linux-4.11.0-2-amd64-x86_64/test-install/pike/8.1.11/lib/modules/Web.pmod/SocketIO.pmod","",0,1)
test-install/pike/8.1.11/lib/master.pike:1603: master()->findprog("/home/hungerf3/xeno/xenoclient/[pike-8](#8008).1/cruncher.ofdoom.com/buildtmp/Pike-v8.1-snapshot/build/linux-4.11.0-2-amd64-x86_64/test-install/pike/8.1.11/lib/modules/Web.pmod/SocketIO.pmod",".pmod",0,1)
test-install/pike/8.1.11/lib/master.pike:1639: master()->low_cast_to_program("/home/hungerf3/xeno/xenoclient/[pike-8](#8008).1/cruncher.ofdoom.com/buildtmp/Pike-v8.1-snapshot/build/linux-4.11.0-2-amd64-x86_64/test-install/pike/8.1.11/lib/modules/Web.pmod/SocketIO",0,0,1)
test-install/pike/8.1.11/lib/master.pike:1917: master()->low_cast_to_object("/home/hungerf3/xeno/xenoclient/[pike-8](#8008).1/cruncher.ofdoom.com/buildtmp/Pike-v8.1-snapshot/build/linux-4.11.0-2-amd64-x86_64/test-install/pike/8.1.11/lib/modules/Web.pmod/SocketIO.pmod",0,0)
test-install/pike/8.1.11/lib/master.pike:2145: Web->low_ind("SocketIO",UNDEFINED)
test-install/pike/8.1.11/lib/master.pike:2223: Web->ind("SocketIO")
test-install/pike/8.1.11/lib/master.pike:2259: Web->`[]("SocketIO")
test-install/pike/8.1.11/lib/master.pike:2443: Web->ind("SocketIO")
test-install/pike/8.1.11/lib/master.pike:2498: Web->`[]("SocketIO")
test-install/pike/8.1.11/lib/master.pike:3139: master()->resolv("Web.SocketIO",UNDEFINED,UNDEFINED)
/home/hungerf3/xeno/xenoclient/[pike-8](#8008).1/cruncher.ofdoom.com/buildtmp/Pike-v8.1-snapshot/src/test_resolve.pike:45: /main()->test_resolv("/home/hungerf3/xeno/xenoclient/[pike-8](#8008).1/cruncher.ofdoom.com/buildtmp/Pike-v8.1-snapshot/build/linux-4.11.0-2-amd64-x86_64/test-install/pike/8.1.11/lib/modules/Web.pmod/SocketIO.pmod",158,UNDEFINED)
/home/hungerf3/xeno/xenoclient/[pike-8](#8008).1/cruncher.ofdoom.com/buildtmp/Pike-v8.1-snapshot/src/test_resolve.pike:134: /main()->test_dir("/home/hungerf3/xeno/xenoclient/[pike-8](#8008).1/cruncher.ofdoom.com/buildtmp/Pike-v8.1-snapshot/build/linux-4.11.0-2-amd64-x86_64/test-install/pike/8.1.11/lib/modules/Web.pmod",158,UNDEFINED)
/home/hungerf3/xeno/xenoclient/[pike-8](#8008).1/cruncher.ofdoom.com/buildtmp/Pike-v8.1-snapshot/src/test_resolve.pike:92: /main()->test_dir("/home/hungerf3/xeno/xenoclient/[pike-8](#8008).1/cruncher.ofdoom.com/buildtmp/Pike-v8.1-snapshot/build/linux-4.11.0-2-amd64-x86_64/test-install/pike/8.1.11/lib/modules",158,UNDEFINED)
/home/hungerf3/xeno/xenoclient/[pike-8](#8008).1/cruncher.ofdoom.com/buildtmp/Pike-v8.1-snapshot/src/test_resolve.pike:158: /main()->main()

No result from subprocess (died of signal SIGABRT)