static unwrap(safeStyleSheet) {
    // Perform additional Run-time type-checking to ensure that
    // safeStyleSheet is indeed an instance of the expected type.  This
    // provides some additional protection against security bugs due to
    // application code that disables type checks.
    // Specifically, the following checks are performed:
    // 1. The object is an instance of the expected type.
    // 2. The object is not an instance of a subclass.
    if (safeStyleSheet instanceof SafeStyleSheet &&
        safeStyleSheet.constructor === SafeStyleSheet) {
      return safeStyleSheet.privateDoNotAccessOrElseSafeStyleSheetWrappedValue_;
    } else {
      fail(
          'expected object of type SafeStyleSheet, got \'' + safeStyleSheet +
          '\' of type ' + goog.typeOf(safeStyleSheet));
      return 'type_error:SafeStyleSheet';
    }
  }
Performs a runtime check that the provided object is indeed a
SafeStyleSheet object, and returns its value.

@param {!SafeStyleSheet} safeStyleSheet The object to extract from.
@return {string} The safeStyleSheet object's contained string, unless
    the run-time type check fails. In that case, `unwrap` returns an
    innocuous string, or, if assertions are enabled, throws
    `asserts.AssertionError`.
@deprecated Use `safevalues.unwrapStyleSheet` combined with `toString()`
    instead.