Dartc: a compile error when implementing two interfaces

[DartBot]

This issue was originally filed by ms...@unipro.ru

---

What steps will reproduce the problem?
Run the following test with dartc

interface S<Type> {}

interface S1 extends S<int> {}

interface S2 extends S<double> {}

class T implements S1, S2 {}

main() {
  Expect.isTrue(new T() is S<int>);
  Expect.isTrue(new T() is S<double>);
  Expect.isTrue(new T() is S<num>);
  Expect.isTrue(new T() is S<Object>);
  Expect.isTrue(new T() is S);
}

What is the expected output? What do you see instead?
Expected: there shouldn't be any errors or warnings.
Actual: compile error: "S<int> and S<double> are duplicated in the supertype graph".

What version of the product are you using? On what operating system?
Dartc, r8026. OS Ubuntu 10.04

Please provide any additional information below.
co19 test: Language/13_Types/4_Interface_Types_A11_t04.dart

[kasperl]

Added Area-Analyzer, Triaged labels.

[danrubel]

Added this to the M1 milestone.

[bwilkerson]

The listed tests are currently passing and there are no tests marked as failing because of this issue.

---

Added AssumedStale label.