null is instance of Dynamic

[ghost]

co19 test:

LangSpecTest/13_Types/13_4_Interface_Types/A07/t02
class CheckDynamic<D> {
  CheckDynamic() {
  }
  
  void check() {
    Expect.isTrue(null is D);
  }
}

main() {
  new CheckDynamic().check();
}

[crelier]

Fixed in r1715.

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Added Fixed label.