firebase-collection: Modifying new locally-pushed items throws

[kevinpschaaf]

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Repro: http://jsbin.com/yuzixa/edit?html,console,output

• Click 'Add'
• Type in input
• Note error

Uncaught Error: Firebase.child failed: First argument was an invalid path: "undefined". Paths must be non-empty strings and can't contain ".", "#", "$", "[", or "]"

This is due to the previous https://github.com/GoogleWebComponents/firebase-element/pull/60 fix which avoided the round-trip modification from the server, which is where the local __firebaseKey__ property is set. Simple fix is to set the new key in add after push, as its synchronously available there.

[GeoloeG-IsT]

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