Strong mode analyzer doesn't like subtype-overridden subclass members

[lexaknyazev]

Dart VM version: 1.13.0-dev.7.4 (Tue Oct 27 15:05:26 2015) on "windows_x64"

import 'dart:async';

abstract class Foo {
  EventSink<int> sink;
}

class BarSink implements EventSink<int> {
  void add(int event) {}
  void addError(errorEvent, [StackTrace stackTrace]) {}
  void close() {}
}

class Bar extends Foo {
  BarSink sink;
}

gives
Error:(14, 3) Invalid override. The type of Bar.sink= ((BarSink) → void) is not a subtype of Foo.sink= ((EventSink<int>) → void).

[vsmenon]

This is intentional, though we really need to improve the message here. The problem is that overriding the setter with a more specific type isn't sound. Code that is just setting the sink field of a Foo with a different EventSink<int> type could inadvertently violate the contract of Bar.

EventSink<int> sink = ...;
Foo f = ...;
f.sink = sink; // Can this ever fail?

Note, this is not a problem with getters. If you declared sink as a getter, you should be fine:

abstract class Foo {
  EventSink<int> get sink;
}

[munificent]

Duplicate of #24928.