Add more class modifiers to dart:collection.

Pure interfaces made `interface`s
implementation classes which cannot/should not be extended made `final`.

A class like `HasHMap` which provides the `Map` interface,
and no implementation except factory constructors
for internal implementations, is made `final`.

Unified {List,Set,Map}{Base,Mixin} into their `Base` class.
Deprecations are retained in comments for now, to be landed
separately. Search for '// TODO: @Deprecated'.

Tested: No new test, only adding restrictions on use.
CoreLibraryReviewExempt: Everybody's on vacation, everybody everywhere.
Change-Id: Ia83b8a3bb20b5b214546b328d4492de6658253db
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/288240
Reviewed-by: Aske Simon Christensen <askesc@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Lasse Nielsen <lrn@google.com>
Reviewed-by: Nate Bosch <nbosch@google.com>