RegExp.toString gives incorrect result without RegExp.flags

[bz2]

The RegExp.toString polyfill creates invalid patterns in the form /a/undefined unless the browser either supports RegExp.flags natively or that is also polyfilled. This is an issue when trying to use targetted imports as created by the @babel/preset-env setting useBuiltIns: 'usage'.

From the compat data, this is a problem for browsers like IE 11 and Edge, as Chrome/Firefox and so on have had native support for a while:
https://github.com/zloirock/core-js/blob/v3.0.1/packages/core-js-compat/src/data.js#L786

Tracking this down took a while, so I created minimal-ish project that reproduces the problem for demonstration:
https://github.com/zegami/test_core_regexp

[bz2]

As a workaround, if a file depend on toString() for RegExp can manually include the core-js import for regexp/flags too:

// TODO: Remove when zloirock/core-js#536 is fixed upstream.
import 'core-js/es/regexp/flags';

This works even if the regexp/to-string polyfill is autocreated by @babel/preset-env.