Fix parsing of non-primitive Expressions

[TimvdLippe]

This came up during parsing of
https://github.com/PolymerElements/iron-form-element-behavior/blob/6087b69013619e5b3e023a10824d25cf7530d1fc/iron-form-element-behavior.html#L74-L84 where conditionals are used instead of direct assignments.

• CHANGELOG.md has been updated

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[googlebot]

So there's good news and bad news.

👍 The good news is that everyone that needs to sign a CLA (the pull request submitter and all commit authors) have done so. Everything is all good there.

😕 The bad news is that it appears that one or more commits were authored by someone other than the pull request submitter. We need to confirm that they're okay with their commits being contributed to this project. Please have them confirm that here in the pull request.

Note to project maintainer: This is a terminal state, meaning the cla/google commit status will not change from this State. It's up to you to confirm consent of the commit author(s) and merge this pull request when appropriate.