Target ES5 not working with angular8

[dklmuc]

Reopen of #22

When simply targeting ES5 in tsconfig, the builded bundle is not ES5-compatible. You see stuff like extends or spread operator in it.

see branch es5 in minimal reproduction: https://github.com/dklmuc/queueing-subject-angular8/tree/es5

[insidewhy]

Reopened original.