fix(angular): do not copy $$hashKey in copy/extend functions.

[spamdaemon]

Copying the $$hashKey as part of copy/extend operations makes little
sense since hashkey is used primarily as an object id, especially in
the context of the ngRepeat directive. This change maintains the
existing $$hashKey of an object that is being copied into (likewise for
extend).
It is not uncommon to take an item in a collection, copy it,
and then append it to the collection. By copying the $$hashKey, this
leads to duplicate object errors with the current ngRepeat.

Closes #1875

[petebacondarwin]

There is already a PR related to this: #2382. Can you collaborate with them to get the best overall solution?
Thanks

[spamdaemon]

I'm not sure whether its a good idea to ignore the $$ properties in either extend or copy. With regards to extend I found at least 1 use in location.js that relied on $$ properties to be copied over.

I'd almost want separate public and private version extend/copy because the use-cases are slightly different.

Btw, I've created a new function startsWith(x,prefix) which is efficient and doesn't create garbage on the heap. It would be nice to make that available via angular.startsWith().

[spamdaemon]

I was wondering if copy($location) should work. If it should work then we cannot drop those $$ properties since they are referenced directly in the public interface of that service.

I think my first patch is semantically more correct and will result in fewer problems down the road.

[spamdaemon]

Any feedback?

[spamdaemon]

I've decided to abandon the second patch and revert back to my initial patch.

[petebacondarwin]

@spamdaemon - so where does this leave us? :-)

[spamdaemon]

You can just merge this patch as far as I'm concerned. I've not heard from anyone whether the patch is fine or not.
I just did not think it was ok to drop the $$ properties in general.
So this patch does 2 things:

1. it does not copy the $$hashKey over in both extend and copy functions
2. it keeps the $$hashKey in the destination object, if it was already set.

[petebacondarwin]

OK, looks good. Merged at 6d0b325 and 016e1e6. Thanks.