Switch pkg packages, pub, and dartdoc to use package: imports.

sdk/lib/_internal/dartdoc/test/markdown_test.dart

Index: sdk/lib/_internal/dartdoc/test/markdown_test.dart
diff --git a/sdk/lib/_internal/dartdoc/test/markdown_test.dart b/sdk/lib/_internal/dartdoc/test/markdown_test.dart
index 44c9431864e83b5a4c1e87031d66eac6b66f63d7..aff43da3dfdeeee4cbd280d64cbf8749edd8c657 100644
--- a/sdk/lib/_internal/dartdoc/test/markdown_test.dart
+++ b/sdk/lib/_internal/dartdoc/test/markdown_test.dart
@@ -5,12 +5,11 @@
 /// Unit tests for markdown.
 library markdownTests;
 
+import 'package:unittest/unittest.dart';
+
 // TODO(rnystrom): Use "package:" URL (#4968).
 import '../lib/markdown.dart';
 
-// TODO(rnystrom): Better path to unittest.
-import '../../../../../pkg/unittest/lib/unittest.dart';
-
 /// Most of these tests are based on observing how showdown behaves:
 /// http://softwaremaniacs.org/playground/showdown-highlight/
 void main() {