Fix incorrect path modification in dom-repeat __handleObservedPaths()

[MajorBreakfast]

Fixes #4983

The path modification is wrong. The path is already modified in __handleItemPath()
https://github.com/Polymer/polymer/blob/master/lib/elements/dom-repeat.html#L649

This causes the sort and filter to fail if a nested property is modified, e.g. 'items.1.prop.nestedProp'. The current code gets the already modified 'prop.nestedProp' and incorrectly modifies it to nestedProp.

@kevinpschaaf

[TimvdLippe]

Please add a regression test so we can verify the original issue remains fixed.

[MajorBreakfast]

There you go.

BTW the test suite for dom-repeat is over 4000 lines and of code. Would it be alright if I broke it up into multiple files in another PR? (not today, but sometime in the future) I'm thinking of one file per custom component and one file per suite. I'm not a big fan of long files because it leads me to waste a lot of time trying to find where I just left off.

[TimvdLippe]

We generally keep it in one file if it concerns one feature. I think it is okay for now to keep it like this, to not overflow the tests directory.

[TimvdLippe]

This should be function() { return [...]; }

[MajorBreakfast]

May I use ES2015 style value () { return []; }?

[MajorBreakfast]

@TimvdLippe I was thinking about a folder ;)

[MajorBreakfast]

safari 11.0.2            Tests passed
firefox 57               Tests passed
chrome 62                Tests passed

[MajorBreakfast]

Ready to merge: It fixes a bug in Polymer that the tests previously didn't cover

[TimvdLippe]

Implementation and test look good. We have to fix #5050 first and then we can merge this one

[MajorBreakfast]

@TimvdLippe Ah okay. Thx for the update :)

[kevinpschaaf]

Awesome fix, @MajorBreakfast and @michalsukupcak. Thanks for the contribution.