anchor.diff:    return UNINTERCEPTED(a + b);
   }
   
  -div(var a, var b) {
  +div$slow(var a, var b) {
     if (checkNumbers(a, b)) {
       return JS('num', @'# / #', a, b);
     }
     return UNINTERCEPTED(a / b);
   }
   
  -mul(var a, var b) {
  +mul$slow(var a, var b) {
     if (checkNumbers(a, b)) {
       return JS('num', @'# * #', a, b);
     }
     return UNINTERCEPTED(a * b);
   }
   
  -sub(var a, var b) {
  +sub$slow(var a, var b) {
     if (checkNumbers(a, b)) {
       return JS('num', @'# - #', a, b);
     }
  
positive.code: #source('native_helper.dart');
  #source('regexp_helper.dart');
  #source('string_helper.dart');
  
  /**
   * Returns true if both arguments are numbers.
  
negative.code:   return UNINTERCEPTED(-a);
  }
  
  index$slow(var a, var index) {
    if (a is String || isJsArray(a)) {
      if (index is !int) {
        if (index is !num) throw new IllegalArgumentException(index);
  