I should be able to supply $scope.$watch a comparator function to check for equality

[roypeled]

Currently, $watch either checks for equality by object reference, or by Angular's equals function, which iterates through the objects properties.

We should be able to supply $watch with an comparator function to have a more lean equality detection.

[pkozlowski-opensource]

@roypeled Yes, this was discussed already. Could you add unit tests to move forward on this one?

[pkozlowski-opensource]

PR Checklist (Minor Feature)

• Contributor signed CLA now or in the past (if you just signed, leave a comment here with your real name for cross reference)
• Feature improves existing core functionality
• API is compatible with existing Angular apis and relevant standards (if applicable)
• PR doesn't contain a breaking change
• PR contains unit tests
• PR contains e2e tests (if suitable)
• PR contains documentation update
• PR passes all tests on Travis (sanity)
• PR passes all tests on ci.angularjs.org (cross-browser compatibility)
• PR is rebased against recent master
• PR is squashed into one commit per logical change
• PR's commit messages are descriptive and allows us to autogenerate release notes (required [commit message

[pkozlowski-opensource]

@roypeled Are you still interested in working on this PR? It will needs tests as a start.

[petebacondarwin]

@roypeled - are you able to improve this PR and have you signed the CLA?

[mhevery]

Closing due to 1) inactivity, 2) No CLA, 3) lack of tests, 4) having a custom comparator also implies that we need to have a custom clone function for the object to get the full benefit of this API change.