Option to supply equality comparer in ObservableList

[mkatch]

ObservableList normalizes change records before sending the change event
https://github.com/dart-lang/observable/blob/5cb19656f0809b961a328b8f7054a0eb14071978/lib/src/observable_list.dart#L330
The actual work is done by the utility function projectListSplices, which has an optional equality parameter, but this parameter is not used by ObservableList. It would be nice to be able to control the equality comparer used for the sake of determining if the elements are equal. I imagine that an optional equality comparer parameter would be added to some of the constructors of ObservableList.

In my particular use case, I expect reference equality semantics for the purpose of determining whether the list has changed, but the concrete item type has an overridden equality operator to perform deep comparison.