Added back abstract class change and fixed test

pkg/docgen/test/single_library_test.dart

 library single_library_test;
 
 import 'dart:io'; 
 
 import 'package:path/path.dart' as path;
 import 'package:unittest/unittest.dart';
 
 import '../lib/docgen.dart';
 
 main() {
@@ skipping 38 matching lines @@
         .then(expectAsync1((mirrorSystem) {
           var testLibraryUri = new Uri(scheme: 'file', 
               path: path.absolute(fileName));
           var library = generateLibrary(mirrorSystem.libraries[testLibraryUri]);
           expect(library is Library, isTrue);
           
           var classTypes = library.classes;
           expect(classTypes is ClassGroup, isTrue);
           
           var classes = [];
-          classes.addAll(classTypes.abstractClasses.values);
-          classes.addAll(classTypes.regularClasses.values);
+          classes.addAll(classTypes.classes.values);
           classes.addAll(classTypes.errors.values);
           expect(classes.every((e) => e is Class), isTrue);
           
           expect(classTypes.typedefs.values.every((e) => e is Typedef), isTrue);
           
           var classMethodTypes = [];
           classes.forEach((e) {
             classMethodTypes.add(e.methods);
             classMethodTypes.add(e.inheritedMethods);
           });
@@ skipping 55 matching lines @@
           expect(methodDocComment == 'test.A.doThis', isTrue);
           
           // Testing something with no reference
           var libraryDocComment = fixReference('foobar', libraryMirror,
               classMirror, methodMirror).children.first.text;
           expect(libraryDocComment == 'foobar', isTrue);
         })).whenComplete(() => temporaryDir.deleteSync(recursive: true));
     });
   });
 }