@Value.Lazy
    TypeAdaptersMirror defaultTypeAdapters() {
      @Nullable TypeElement typeElement =
          findElement(TypeAdaptersMirror.qualifiedName());

      Preconditions.checkState(typeElement != null,
          "Processor internal error, @%s is not know to be on the classpath",
          TypeAdaptersMirror.qualifiedName());

      return TypeAdaptersMirror.from(typeElement);
    }
Default type adapters should only be called if {@code Gson.TypeAdapters} annotation is
definitely in classpath. Currently, it is called by for mongo repository module,
which have {@code gson} module as a transitive dependency.
@return default type adapters
value-processor/src/org/immutables/value/processor/meta/Proto.java