Add language-level support for object forwarding

[DartBot]

This issue was originally filed by @yissachar

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Object delegation is a common solution when inheritance is not an option. For instance, before ListBase was introduced, one option to get around inheriting List was to use a delegate list and then forward all the method calls. Forwarding the method calls can be done manually, which is tedious. Alternatively, forwarding can quickly be accomplished with noSuchMethod at the cost of tooling support (since the analyzer no longer can verify your calls) and potential code size issues when deploying to JavaScript.

Language-level support for this common pattern would provide the advantages of manual delegation without forcing the user to type out every method.

One possible implementation would be through a "proxy" keyword or something similar that would indicate a field is to be delegated.

class MyList {
  proxy _list = [0, 1, 2];
}

var myList = new MyList();
myList.add(3);
print(myList.length);

Multiple proxies for a class would not be allowed if they would cause ambiguity, but could be allowed otherwise (e.g. proxying two Lists would not be allowed since there would be no way to determine which to delegate to, but proxying List alongside Foo which does not share any method signatures would be fine).

[dgrove]

Added Area-Language, Triaged labels.

[gbracha]

There is some question if we are talking about delegation or just forwarding. It sounds like forwarding in this case.

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Set owner to @gbracha.
Added Accepted label.
Changed the title to: "Add language-level support for object forwarding".

[DartBot]

This comment was originally written by @yissachar

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Yes, I mean forwarding only. Sorry for confusing the terms.

[Aetet]

How this issue can be solved with existing tools at dart? Is it possible?

[natebosch]

Unfortunately I didn't see this issue when I filed dart-lang/language#3748 with a very similar request. Since that issue has more discussion I think we should use it to track any decisions on this going forward.