Fix issue with conditional expressions

[arv]

For conditionals we failed to setup the observer for both branches
as required. This lead to us only observing either the true or false
branch but not both.

Fixes #40

[arv]

@rafaelw FYI

[jmesserly]

LGTM. Another approach might be to only observe the live branch (either true or false) then if the value of "test" changes, observe the other branch. Seems a bit more complex to code that up though, so probably not worth it. Not sure if there are semantic reasons to avoid evaluating the non-live branch. I wondered about && and ||, but it looks like they are treated as generic binary operators without short circuiting: https://github.com/Polymer/polymer-expressions/blob/master/src/polymer-expressions.js#L290

[rafaelw]

Good catch, but I don't think this quite right.

In particular, if whole expression is dynamic (deps) and either branch contains the dynamic sub-expression, this will fails to correctly re-capture deps.

It seems to me the most correct semantics would be:

1. That expressions with conditionals are marked as dynamic -- and they must re-collect deps when the test flips between truthy & falsy.

2. Only the branch which is currently active ever gets eval'd. This will avoid surprising errors.

I think an acceptable near-term fix would be to eval test, & both branches IFF |observer| is defined (which means that deps are being collected). Obviously, this fails (2), but we can open a separate bug for that.

[arv]

@rafaelw Do you want me to make || and && short circuit as well?

I think the best approach for now is probably to always evaluate all branches. We can look into making ?:, || and && only evaluate the needed branch in another PR.

[arv]

I marked them as dynamic and made them all lazy. PTAL

[rafaelw]

lgtm