refactor(multiple): resolve rxjs 7 compilation errors

[crisbeto]

Fixes a few places that will become compilation errors in rxjs version 7. All of the changes are backwards-compatible so we can merge them now so that we have less issues to resolve once we make the switch.

Note: I'm marking this as merge safe, because all of the changes are internal and they only affect type information. There are a couple of extra null checks, but they're in cdk-experimental which is also merge safe.

[mmalerba]

LGTM

[jelbourn]

I have to be a wet blanket and remove merge safe because this does touch sources used in Google. Even though the types should be internal only, they still might break due to stuff like different build setup, rxjs local mods, etc.

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