anchor.diff:    toString() => 'invoke interceptor: ${element.name}';
     accept(HVisitor visitor) => visitor.visitInvokeInterceptor(this);
   
  -  
     String get builtinJsName() {
       if (getter
           && name == const SourceString('length')
  
positive.code:   HType computeType() {
      HType computedType = computeInputsType();
      if (computedType.isConflicting() && left.isString()) return HType.STRING;
      if (!computedType.isUnknown()) return computedType;
      if (left.isNumber()) return HType.NUMBER;
      return HType.UNKNOWN;
    }
  
negative.code: 
    HType computeType() {
      HType inputsType = computeInputsType();
      if (inputsType.isKnown()) return inputsType;
      if (left.isNumber()) return HType.NUMBER;
      return HType.UNKNOWN;
    }
  