Previously, the correct behavior of Angular custom elements relied on
the constructor being called (and thus the `injector` property being
initialized). However, some polyfills (e.g. `document-register-element`)
do not call the constructor of custom elements, which resulted in the
`injector` property being undefined and the `NgElementStrategy` failing
to be instantiated.
This commit fixes it by being tolerant to the `injector` property being
undefined and falling back to the injector passed to the
`createCustomElement()` config.
NOTE:
We don't have proper tests exercising the situation where the
constructor is not called. For now this is tested using a Google
internal test suite (which is how this issue was caught).
This commit also adds a rudimentary unit test to emulate this situation.
PR Close#36114
89b44d1900