anchor.diff:          builder.attribute('lang', 'en', namespace: uri);
           builder.element('element', namespace: uri);
         }, namespace: uri);
  -      var actual = builder.build().toString();
  +      var xml = builder.build();
  +      assertTreeInvariants(xml);
  +      var actual = xml.toString();
         var expected =
             '<schema xmlns="http://www.w3.org/2001/XMLSchema" lang="en">'
               '<element />'
  
positive.code:         builder.attribute('lang', 'en', namespace: uri);
          builder.element('element', namespace: uri);
        }, namespace: uri);
        var actual = builder.build().toString();
        var expected =
            '<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema" xsd:lang="en">'
              '<xsd:element />'
  
negative.code:           builder.element('special');
          });
        });
        var xml = builder.build();
        assertTreeInvariants(xml);
        var actual = xml.toString();
        var expected =
            '<?xml encoding="UTF-8"?>'
            '<bookstore>'
  