code: function getSafeUri(uri, propName, uriRewriter) {
    if (!uriRewriter) {
      return null;
    }
    var safeUri = uriRewriter(uri, propName);
    if (safeUri && SafeUrl.unwrap(safeUri) != SafeUrl.INNOCUOUS_STRING) {
      return 'url("' +
          SafeUrl.unwrap(safeUri).replace(NORM_URL_REGEXP, normalizeUrlChar) +
          '")';
    }
    return null;
  }
docs: Constructs a safe URI from a given URI and prop using a given uriRewriter
  function.
  @param {string} uri URI to be sanitized.
  @param {string} propName Property name which contained the URI.
  @param {?function(string, string):?SafeUrl} uriRewriter A URI rewriter that
  returns a {@link SafeUrl}.
  @return {?string} Safe URI for use in CSS.