code: static JSType createGenericType(
        boolean shouldCreate, JSTypeRegistry registry, JSTypeNative typeName, JSType typeArg) {
      if (!shouldCreate) {
        return null;
      }
      ObjectType genericType = (ObjectType) (registry.getNativeType(typeName));
      ObjectType uninstantiated = genericType.getRawType();
      return registry.instantiateGenericType(uninstantiated, ImmutableList.of(typeArg));
    }
docs: Returns the JSType as specified by the typeName and instantiated by the typeArg.
  Returns null if shouldCreate is false.