test generated Dart code

Question

Prolog

I use source_gen to generate Dart code. I'd like to test the output of my generator (using test package). I looked at the tests of source_gen and used json_serializable_test.dart as a template. I can call generate on my generator and get the result as a String. Now I'd like to test, if classes and methods are generated as I expect. Sadly, this kind of test is missing in json_serializable_test.dart:

expect(output, isNotNull);

// TODO: test the actual output
// print(output);

I modified the helpers (like _getCompilationUnitForString) to pass a source (instead of always using _testSource) and get its analyzer elements. Thereby, I can specify the input and expected output of my generator as a file or string and get the analyzer elements of input, output and expected output.

Approach

I came up with this primitive approach to match classes by their name and field declarations:

import 'package:analyzer/src/generated/element.dart';
import 'package:source_gen/src/utils.dart';
import 'package:test/test.dart';


Matcher equalsClass(expected) => new ClassMatcher(expected);

class ClassMatcher extends Matcher {
  final ClassElementImpl _expected;

  const ClassMatcher(this._expected);

  @override
  Description describe(Description description) => description
      .add('same class implementation as ')
      .addDescriptionOf(_expected);

  @override
  Description describeMismatch(ClassElementImpl item, Description description,
                               Map matchState, bool verbose) => description
      .add('${friendlyNameForElement(item)}')
      .add(' differs from ')
      .add('${friendlyNameForElement(_expected)}');

  @override
  bool matches(ClassElementImpl item, Map matchState) {
    return (item.displayName == _expected.displayName) &&
        unorderedEquals(_mapFields(item.fields))
            .matches(_mapFields(_expected.fields), matchState);
  }
}

Iterable _mapFields(List<FieldElement> fields) => fields
    .map(friendlyNameForElement);

This solution might be error-prone, since I compare fields by a String representation. Apart from this, the mismatch desciption is very poor. How can I improve my matcher?

Ps: Is there a better way to compare generated code with expectations?