Loosen undefined rule for computational expressions

[kaste]

• Let developers opt-in to see undefined's in observers et.al.
• Let computed properties and annotations evaluate to undefined if their dependencies cannot be fulfilled

Can be seen as an add-on to #3423, could be applied individually as well. All in all non breaking change.

Ref issue #3453, fixes #2158.

[kaste]

I'm not convinced anymore that this is a good change. I actually doubt it is. Feel free to close it.
-- Sorry wrong PR.

[TimvdLippe]

Observers run with undefined in Polymer 2 as described in the upgrade guide, therefore I am closing this PR. Thanks for your suggestions, they helped defining the Polymer 2 API 😄