Collection transformations should return "views" not "snapshots"

[DartBot]

This issue was originally filed by @seaneagan

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The following methods transform one Collection into another:

Map#getKeys
Map#getValues
Collection#filter
Collection#map (see issue #945)

At least some of these currently return an eager "snapshots" of the transformation.

They should instead return lazy "views" backed by the original Collection. Here's why:

* Avoids copying internal values (which is O(n))

* Can chain together multiple transformations without creating intermediate throw away Collections.

* Avoid having to call the method every single time updated values from the original Collection are needed (the majority use case).

* Copy constructors are already a much more orthogonal way to snapshot Collections, on the rare occasion that that is needed. For example:

// see issue #1248
new Set.from(map.keys)
new List.from(map.values)

// see issue #1304
new Collection.from(collection.filter(callback))
new Collection.from(collection.map(callback))

[dgrove]

Removed Type-Defect label.
Added Type-Enhancement, Area-Library, Triaged labels.

[lrhn]

They return views now.

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Added Fixed label.