(v6.x backport) events,test: fix TypeError in EventEmitter warning
Allows Symbol to be converted to String so it can be included in the error.
Fixes: https://github.com/nodejs/node/issues/9003
Conflicts: lib/events.js
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines
Affected core subsystem(s)
src, test