public ListenableFuture<V> apply(X t) {
        throw newAssertionError("Unexpected fallback", t);
      }
    };
  }

  /** Alternative to AssertionError(String, Throwable), which doesn't exist in GWT 2.6.1. */
  private static AssertionError newAssertionError(String message, Throwable cause) {
    AssertionError e = new AssertionError(message);
    e.initCause(cause);
    return e;
  }

      public ListenableFuture<V> apply(X t) {
        throw newAssertionError("Unexpected fallback", t);
      }
    };
  }

  /** Alternative to AssertionError(String, Throwable), which doesn't exist in GWT 2.6.1. */
  private static AssertionError newAssertionError(String message, Throwable cause) {
    AssertionError e = new AssertionError(message);
    e.initCause(cause);
    return e;
  }

// TODO(dpb): Consider reusing one CloseableList for the entire pipeline, modulo combinations.
@DoNotMock("Use ClosingFuture.from(Futures.immediate*Future)")
@J2ktIncompatible
@ElementTypesAreNonnullByDefault
// TODO(dpb): GWT compatibility.
public final class ClosingFuture<V extends @Nullable Object> {

  private static final LazyLogger logger = new LazyLogger(ClosingFuture.class);

  /**