code: private ImmutableList<ObjectType> resolveTypeListHelper(
        ImmutableList<ObjectType> list, ErrorReporter reporter) {
      boolean changed = false;
      ImmutableList.Builder<ObjectType> resolvedList = ImmutableList.builder();
      for (ObjectType type : list) {
        JSType rt = type.resolve(reporter);
        if (!rt.isObjectType()) {
          reporter.warning(
              "not an object type: " + rt + " (at " + toString() + ")",
              source.getSourceFileName(),
              source.getLineno(),
              source.getCharno());
          continue;
        }
        ObjectType resolved = rt.toObjectType();
        resolvedList.add(resolved);
        changed |= (resolved != type);
      }
      return changed ? resolvedList.build() : null;
    }
docs: Resolve each item in the list, and return a new list if any references changed. Otherwise,
  return null.