Previously, when `ngcc` was reflecting on class members it did not
account for the fact that a member could be of the kind
`IndexSignature`. This can happen, for example, on abstract classes (as
is the case for [JsonCallbackContext][1]).
Trying to reflect on such members (and failing to recognize their kind),
resulted in warnings, such as:
```
Warning: Unknown member type: "[key: string]: (data: any) => void;
```
While these warnings are harmless, they can be confusing and worrisome
for users.
This commit avoids such warnings by detecting class members of the
`IndexSignature` kind and ignoring them.
[1]: https://github.com/angular/angular/blob/4659cc26e/packages/common/http/src/jsonp.ts#L39
PR Close#33198
78214e72ea