Don't assume global

Angular CLI no longer supports global shims as of 6.x. Therefore this module will break projects which use it:
https://github.com/inspect-js/has-symbols/blob/aa57a17b19708906d1927f821ea8e73394d84ca4/index.js#L3

See also: https://github.com/ljharb/es-abstract/issues/59 and https://github.com/angular/angular-cli/issues/9827#issuecomment-369578814