code: public static SanitizedContent jsIdentifier(String identifier) {
      checkArgument(
          VALID_JS_IDENTIFIER_PATTERN.matcher(identifier).matches(),
          "JS identifier '%s' should match the pattern '%s'",
          identifier,
          VALID_JS_IDENTIFIER_PATTERN.pattern());
      checkArgument(
          !INVALID_JS_IDENTIFIERS.contains(identifier),
          "JS identifier '%s' should not be a reserved word or match a literal",
          identifier);
      return UnsafeSanitizedContentOrdainer.ordainAsSafe(identifier, ContentKind.JS);
    }
docs: Validates that {@code identifier} matches a safe pattern for JS identifiers and ordains the
  value as JS.
  
  <p>TODO: this appears to be redundant with some code in JsSrcUtils.