Add default methods to allow evolutions on classes

[a14n]

I really like Dart implicit interface (every classes can be used as interface) but it makes harder to make evolutions on classes.
For instance it's almost impossible today to add a method to Map or List without releasing a major version.
In Java, interfaces can evolve with the addition of default methods. Those methods are implemented only based on the interface methods.

I wondered if something like that could be done in Dart.

For instance, we could add a method groupBy on Map :

class Map<K,V> {
  // new method that will be used by classes implementing Map without defining groupBy
  default Map<K,List<E>> groupBy<K>(K key(E element)) {
    // my implementation that does not used private members
  }
}

class MyMap<K,V> implements Map<K,V> {
}

and my class MyMap that implements Map would have the new method without breakage. It would break only if the implementation as a method with the same name but with different parameters.

WDYT?

[matanlurey]

+1. We strongly need either this and/or something like C#'s extension methods.

Otherwise it is too painful to evolve any library, which leads to a lot of fragmentation.

[eernstg]

In the language team we have discussed adding support for default methods for a while, using exactly the same motivation as given here (to allow widely used interfaces to grow). I've added a reference to this issue in an internal language design issue, to ensure that we're aware of this input on the matter.

[matanlurey]

The lack of something like this just made a ~5 minute task into a ~5 hour one :(

[a14n]

See dart-lang/language#884

[lrhn]

Yep, deferring this to the language repo issue.