Improve how we handle packages/ HTML imports. This resolves better any valid

pkg/polymer/example/canonicalization2/lib/g.html

Index: pkg/polymer/example/canonicalization2/lib/g.html
diff --git a/pkg/polymer/example/canonicalization/lib/a.html b/pkg/polymer/example/canonicalization2/lib/g.html
similarity index 51%
copy from pkg/polymer/example/canonicalization/lib/a.html
copy to pkg/polymer/example/canonicalization2/lib/g.html
index c359e83465506fc711d423057b3452409b44df20..1b9e8d200950ad2a2b310520c3b64d0bc110e25c 100644
--- a/pkg/polymer/example/canonicalization/lib/a.html
+++ b/pkg/polymer/example/canonicalization2/lib/g.html
@@ -3,4 +3,8 @@ Copyright (c) 2013, 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.
 -->
-<script type='application/dart' src='a.dart'></script>
+<!--
+Because this file is under lib/, this URL is broken. It should be either
+'b.html' or '../../packages/canonicalization/b.html'.
+ -->
+<link rel="import" href="../packages/canonicalization/b.html">