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

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

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// 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() {
   group('Paragraphs', () {
     validate('consecutive lines form a single paragraph', '''
         This is the first line.
         This is the second line.
         ''', '''
         <p>This is the first line.
         This is the second line.</p>
@@ skipping 880 matching lines @@
 
     // If one string runs out of non-ignored strings, the other must too.
     if (i == a.length) return j == b.length;
     if (j == b.length) return i == a.length;
 
     if (a[i] != b[j]) return false;
     i++;
     j++;
   }
 }