wire __runtime_instanceof to 'instanceof' on upcasts

also adds runtime.instanceOf that can be exported for use by the host

tests/compiler/runtime/instanceof.ts

@@ -1,4 +1,5 @@
 import { __runtime_id, __runtime_instanceof } from "runtime";
+import "../gc/_dummy";
 
 class Animal {}
 class Cat extends Animal {}
@@ -59,3 +60,53 @@ assert(! // Cat isn't necessarily a BlackCat
     __runtime_id<BlackCat>()
   )
 );
+
+var animal: Animal = new Animal();
+var cat: Animal = new Cat();
+var blackcat: Animal = new BlackCat();
+
+assert(animal instanceof Animal); // static true
+assert(!(animal instanceof Cat)); // dynamic false
+assert(!(animal instanceof BlackCat)); // dynamic false
+
+assert(cat instanceof Animal); // static true
+assert(cat instanceof Cat); // dynamic true
+assert(!(cat instanceof BlackCat)); // dynamic false
+
+assert(blackcat instanceof Animal); // static true
+assert(blackcat instanceof Cat); // dynamic true
+assert(blackcat instanceof BlackCat); // dynamic true
+
+var nullableAnimal: Animal | null = new Animal();
+var nullableCat: Animal | null = new Cat();
+var nullableBlackcat: Animal | null = new BlackCat();
+
+assert(nullableAnimal instanceof Animal); // static true
+assert(!(nullableAnimal instanceof Cat)); // dynamic false
+assert(!(nullableAnimal instanceof BlackCat)); // dynamic false
+
+assert(nullableCat instanceof Animal); // static true
+assert(nullableCat instanceof Cat); // dynamic true
+assert(!(nullableCat instanceof BlackCat)); // dynamic false
+
+assert(nullableBlackcat instanceof Animal); // static true
+assert(nullableBlackcat instanceof Cat); // dynamic true
+assert(nullableBlackcat instanceof BlackCat); // dynamic true
+
+var nullAnimal: Animal | null = null;
+var nullCat: Animal | null = null;
+var nullBlackcat: Animal | null = null;
+
+assert(!(nullAnimal instanceof Animal)); // static false
+assert(!(nullAnimal instanceof Cat)); // dynamic false
+assert(!(nullAnimal instanceof BlackCat)); // dynamic false
+
+assert(!(nullCat instanceof Animal)); // static false
+assert(!(nullCat instanceof Cat)); // dynamic false
+assert(!(nullCat instanceof BlackCat)); // dynamic false
+
+assert(!(nullBlackcat instanceof Animal)); // static false
+assert(!(nullBlackcat instanceof Cat)); // dynamic false
+assert(!(nullBlackcat instanceof BlackCat)); // dynamic false
+
+@start export function main(): void {}