Add DelegatingMapSet to collection package

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="96" height="96"hspace="10"> Issue by nex3
Originally opened as dart-lang/sdk#18705

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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.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Set owner to @nex3.
Added Started label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/4865287?v=3" align="left" width="48" height="48"hspace="10"> Comment by lrhn

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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).

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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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".

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Fixed in dart-lang/sdk@39ba636, to be released in collection 0.9.3.

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Added Fixed label.