fix(change_detection): should properly support objects with equality

[pavelgj]

Fixes #670

[chalin]

I know that this is just test code, but would this not make more sense as a static member of FooBar?

[pavelgj]

Done.

[chalin]

Hmm, very interesting fix. But I thought that equality was not taken into consideration in the new CDA --- e.g., see Misko's comment to this post. I am still trying to learn how the new CDA code works and it must be subtle, but I am not seeing how the slight change you made influences how the FooBar class equality is used by the CDA. If you happen to have 2 min at some point and would be willing to explain why your fix works, that would be appreciated. (By the way, I am not doubting the correctness of the fix, I just feel that there is something important in that fix that is important to understand! :)

[caitp]

We don't have operator overloading in ES6, so the rationale for this fix might not matter there (without adding some opinionated method name used for equality within CD, which doesn't really make sense) --- but what seems to be changing is operation ordering. Would that apply to the JS port? Seems like that's potentially still an issue for us.

[chalin]

@pavelgj , nm. Is see that the FooBar equality & hashCode affect map lookup in DuplicateMap (of dirty_checking_change_detector.dart).