Use "package:" imports for pub.

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Review URL: https://codereview.chromium.org//1153643002
diff --git a/bin/pub.dart b/bin/pub.dart
index e781f94..ede1b42 100644
--- a/bin/pub.dart
+++ b/bin/pub.dart
@@ -2,7 +2,7 @@
 // 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.
 
-import '../lib/src/command_runner.dart';
+import 'package:pub/src/command_runner.dart';
 
 void main(List<String> arguments) {
   new PubCommandRunner().run(arguments);