@Override
                  protected Set<String> create(String[] elements) {
                    ImmutableSet.Builder<String> builder = ImmutableSet.builder();
                    builder.forceJdk();
                    builder.add(elements);
                    return builder.build();
                  }
                })
            .named(ImmutableSetTest.class.getName() + ", with JDK builder")

        impl = EmptySetBuilderImpl.instance();
      }
    }

    Builder(@SuppressWarnings("unused") boolean subclass) {
      this.impl = null; // unused
    }

    @VisibleForTesting
    void forceJdk() {
      requireNonNull(impl); // see the comment on the field
      this.impl = new JdkBackedSetBuilderImpl<>(impl);
    }

    final void copyIfNecessary() {
      if (forceCopy) {
        copy();

          <show>package</show>
        </configuration>
      </plugin>
      <!-- Disable "normal" testing, which doesn't work for GWT tests. -->
      <plugin>