Remove HashMap/HashSet (and even Hashable)

[DartBot]

This issue was originally filed by @tomyeh

---

The difference between Map ans HashMap is too subtle. Moreover, it causes unnecessary confusion to Java programmers.

It is better to remove HashMap/HashSet to make the API simpler and more concise.

[iposva-google]

Added Area-Library, Triaged labels.

[lrhn]

I think we should have a plain Map interface, and implementation classes like HashMap, OrderedHashMap, and eventually also other map implementations like TreeMap or SplayMap.

Map has constructors that forward to OrderedHashMap only for convenience. They correspond to Map literals, so if you don't care what you use, just use new Map and it'll work.

[lrhn]

We are sticking with the factory constructors on Map.
They currently return a HashMap. We still consider changing that to a LinkedHashMap, so "{}" and "new Map()" have the same type.
If you care about the type of Map you use, you can use that directly, and if you don't care, just use "new Map()".may

---

Set owner to @lrhn.
Removed Type-Defect label.
Added Type-Enhancement, NotPlanned labels.