Does Dart 1.13 relax superinterface requirements for mixin applications?

Question

In the following code from Mixins in Dart, an application of mixin K results in no warning, despite missing the superinterface method twice():

class S {
  twice(int x) => 2 * x;
}

abstract class I {
  twice(x);
}

abstract class J {
  thrice(x);
}

class K extends S implements I, J {
  int thrice(x) => 3* x;
}

class B {
  twice(x) => x + x;
}

class A = B with K;

class D {
  double(x) => x+x;
}

// E missing 'twice', thus not conforming to
// superinterface S of K. Yet no warning in
// checked mode under Dart 1.13.
class E = D with K;

main() {
  /// A conforms to superinterface S of K
  assert(new A().twice (1) == 2);
  assert(new A().thrice(1) == 3);

  /// E conforms to superinterface S of K
//assert(new E().twice (1) == 2);  // NoSuchMethod 'twice'
  assert(new E().thrice(1) == 3);  // thrice() Provided by K
}

I'm mainly just wanting to fill in some holes in my understanding, so would appreciate someone pointing out any misuse of terms or bad concepts in my question here.

Answer 1

There were changes to how mixins work in Dart version 1.13. It's implemented in the VM, but not yet in dart2js, and I'm not sure about the analyzer.

Still, E is a non-abstract class that doesn't implement its own interface (implements I but doesn't have a non-abstract twice member). That should cause a warning, mixins or no mixins.

On top of that, K extends S, so when using K as a mixin, the mixin application's superclass should also implement S which D doesn't. That should give another warning.