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

pkg/polymer/lib/src/build/import_inliner.dart

 // 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.
 
 /** Transfomer that inlines polymer-element definitions from html imports. */
 library polymer.src.build.import_inliner;
 
 import 'dart:async';
 import 'dart:convert';
 
@@ skipping 164 matching lines @@
 
   _newUrl(String href, Span span) {
     var uri = Uri.parse(href);
     if (uri.isAbsolute) return href;
     if (!uri.scheme.isEmpty) return href;
     if (!uri.host.isEmpty) return href;
     if (uri.path.isEmpty) return href;  // Implies standalone ? or # in URI.
     if (path.isAbsolute(href)) return href;
 
     var id = resolve(sourceId, href, transform.logger, span);
+    if (id == null) return href;
     var primaryId = transform.primaryInput.id;
 
     if (id.path.startsWith('lib/')) {
       return 'packages/${id.package}/${id.path.substring(4)}';
     }
 
     if (id.path.startsWith('asset/')) {
       return 'assets/${id.package}/${id.path.substring(6)}';
     }
 
@@ skipping 23 matching lines @@
   'cite',       // in blockquote, del, ins, q
   'data',       // in object
   'formaction', // in button, input
   'href',       // in a, area, link, base, command
   'icon',       // in command
   'manifest',   // in html
   'poster',     // in video
   'src',        // in audio, embed, iframe, img, input, script, source, track,
                 //    video
 ];