Fix package docnames for select return types, and some more docgen refactoring.

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';
 
 const String DART_LIBRARY = '''
@@ skipping 86 matching lines @@
           expect(variables.every((e) => e is Variable), isTrue);
 
           /// Testing fixReference
           // Testing Doc comment for class [A].
           var libraryMirror = mirrorSystem.libraries[testLibraryUri];
           var classMirror = libraryMirror.classes.values.first;
           var classDocComment = library.fixReference('A').children.first.text;
           expect(classDocComment, 'test.A');
 
           // Test for linking to parameter [A]
-          var method = getDocgenObject(classMirror.methods['doThis']);
+          var method = Indexable.getDocgenObject(classMirror.methods['doThis']);
           var methodParameterDocComment = method.fixReference(
               'A').children.first.text;
           expect(methodParameterDocComment, 'test.A.doThis.A');
 
           // Testing trying to refer to doThis function
           var methodDocComment = method.fixReference(
               'doThis').children.first.text;
           expect(methodDocComment, 'test.A.doThis');
 
           // Testing something with no reference
           var libraryDocComment = method.fixReference('foobar').text;
           expect(libraryDocComment, 'foobar');
         })).whenComplete(() => temporaryDir.deleteSync(recursive: true));
     });
   });
 }