Add DelegatingMapSet to collection package

[nex3]

Suppose there's a collection of objects that are indexed by properties on the objects themselves. Normally this would be represented by a Map, but in some cases it may be more useful to treat it as a Set. This is possible since the key is determinable from each value, so for example "add" could be implemented as "super[keyForValue(value)] = value". It would be nice to add a class to support this to the collection package.

[nex3]

Set owner to @nex3.
Added Started label.

[lrhn]

You can do that using:

     new HashSet(equals: (a, b) => keyForValue(a) == keyForValue(b),
                 hash: (a) => keyForValue(a).hashCode);

Is there something a specialized map-backed-set would be able to do better than this?

If the keyForValue computation is expensive, I can see a reason to extract it once, and use it as the key of a map, but it's going to be used on all lookups as well, so it should probably be cached on the object instead (or maybe in an Expando).

[nex3]

The goal is to provide a Set view of a map's keys, so it's important that the resulting set delegates to the backing map.

For example, in Barback we have a notion of assets that have ids. We store these assets in a Map<AssetId, Asset>, but sometimes it's useful to view this as a Set—it makes it easier to add and remove Assets, for example. In this case, keyForValue would be "(asset) => asset.id".

[nex3]

Fixed in r36447, to be released in collection 0.9.3.

---

Added Fixed label.

[DartBot]

This issue has been moved to dart-lang/collection#11.