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

pkg/docgen/test/single_library_test.dart

Index: pkg/docgen/test/single_library_test.dart
diff --git a/pkg/docgen/test/single_library_test.dart b/pkg/docgen/test/single_library_test.dart
index ed30818fa9033c37e42ae9768163e4f5c4f53f91..3ce19a34aec0565aeb0a4587b5ce3a63bcccee5b 100644
--- a/pkg/docgen/test/single_library_test.dart
+++ b/pkg/docgen/test/single_library_test.dart
@@ -104,7 +104,7 @@ main() {
           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');