Merge pull request #648 from quicktype/save-types-added

Do actually keep track of types added

src/TypeBuilder.ts

@@ -811,7 +811,7 @@ export class UnionAccumulator<TArray, TClass, TMap> implements UnionTypeProvider
 }
 
 export class TypeRefUnionAccumulator extends UnionAccumulator<TypeRef, TypeRef, TypeRef> {
-    private readonly _typesAdded: Set<Type> = Set();
+    private _typesAdded: Set<Type> = Set();
 
     // There is a method analogous to this in the IntersectionAccumulator.  It might
     // make sense to find a common interface.
@@ -819,6 +819,7 @@ export class TypeRefUnionAccumulator extends UnionAccumulator<TypeRef, TypeRef,
         if (this._typesAdded.has(t)) {
             return emptyTypeAttributes;
         }
+        this._typesAdded = this._typesAdded.add(t);
 
         const attributes = t.getAttributes();
         let unionAttributes: TypeAttributes | undefined = undefined;