[dart-misc] Dart 2 Breaking Change: Removing Maps class

['Samuel Rawlins' via Dart Misc]

*What is changing?*

As part of the migration of the Dart core libraries to Dart 2, we are
removing the Maps class. No, not the Map class :), the Maps
<https://api.dartlang.org/stable/1.24.3/dart-collection/Maps-class.html> class,
which provided static methods to help developers `implement Map`.

*What will break?*

If you have code which calls a static Maps method, such as Maps.mapToString
<https://api.dartlang.org/stable/1.24.3/dart-collection/Maps/mapToString.html>,
you will need to change this code.

*How do I fix it?*

Typical usage of the Maps class comes as a class which implements Map and
relies on static Maps methods for implementation. Such code should be
changed to `extend MapBase
<https://api.dartlang.org/dev/2.0.0-dev.22.0/dart-collection/MapBase-class.html>`
or extend `with MapMixin
<https://api.dartlang.org/dev/2.0.0-dev.22.0/dart-collection/MapMixin-class.html>`,
which gives you most of the Map methods for free, by just implementing 5
methods that the rest can call upon: keys, operator[], operator[]=, remove
and clear.

*Note:* Extending MapBase or Mixing in MapMixin instead of implementing Map
is a good practice. It future proofs your code against new methods on the
Map class; you won't need to race to add new method implementations in
order to maintain your `implement Map` contract.

Quick example:

```
class MyMap implements Map {
...

V putIfAbsent(String key, V ifAbsent()) {
_mySanityCheck(key);
return Maps.putIfAbsent(this, key, ifAbsent)
}
```

to


```
class MyMap extends MapBase {
...

V putIfAbsent(String key, V ifAbsent()) {
_mySanityCheck(key);
return super.putIfAbsent(key, ifAbsent)
}
```

*Note 2:* Maps.mapToString
<https://api.dartlang.org/stable/1.24.3/dart-collection/Maps/mapToString.html>
is the one static method that Maps provided which is *not* replaced by a
member method of MapBase or MapMixin. This static method has instead been
moved to the MapBase class, still as a static method, in Dart
2.0.0-dev.22.0. Developers need their own replacement (example
<https://github.com/dart-lang/collection/commit/5943e1681204250f33a833eb5550f270357ad6c8>)
or can bump their packages' minimum SDK version to >= 2.0.0-dev.22.0 and
use MapBase.mapToString
<https://api.dartlang.org/dev/2.0.0-dev.22.0/dart-collection/MapBase/mapToString.html>
.

*Why is this change being made?*

The Maps class is a very old hold-over from before Dart featured mixins! As
the class is implemented now, none of the methods use type parameters, and
so none of them play well with the Dart 2 type system (you would be writing
`as` all over, yuck). Rather than upgrade and maintain this redundant
class, we have opted to delete it in favor of the MapMixin implementations.

[tatumizer-v0.2]

Once you are at it, maybe you can consider merging 'collection' package
into collection library? It has some good stuff in it, which logically
belongs to dart:collection.
Also, there's some general purpose collection-related section in quiver.
All 3 came from the same dart team, not clear what motivation for keeping
these things separate is.