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;
}
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.