language/inference/inconsistent_inheritance_test fails

[scheglov]

class A {
  void f(Object x) {}
}

class B {
  void f(String x) {}
}

class C extends A implements B {
  void f(x) {
    // Infers Object for x.
    Object y = x;
    String z = x;
    //         ^
    // [analyzer] STATIC_TYPE_WARNING.INVALID_ASSIGNMENT
    // [cfe] A value of type 'Object' can't be assigned to a variable of type 'String'.
  }
}

This requires dart-lang/language#975 finished and landed.

[scheglov]

The language change was implemented in 2485242.
b7efcd2 added new error.