Type-check external objects?

[Martin Sjölund]

​https://trac.modelica.org/Modelica/ticket/577

model M
  class A
    extends ExternalObject;
    function constructor input Real r; output A a; end constructor;
    function destructor input A a; end destructor;
  end A;
  class B
    extends ExternalObject;
    function constructor input Real r; output B b; end constructor;
    function destructor input B b; end destructor;
  end B;
  function f
    input A a;
    output Real r = 1.5;
  end f;
  A a = A(time);
  B b = B(time);
  Real r1 = f(a);
  Real r2 = f(b);
end M;