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

pkg/polymer/example/canonicalization/test/dev_test.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.
 
 /** Tests how canonicalization works during development. */
 library canonicalization.dev_test;
 
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'package:polymer/polymer.dart';
@@ skipping 15 matching lines @@
       'initPolymer picks the "package:" url as the canonical url for script '
       'tags whose library is also loaded with a "package:" url.');
     expect(b, 1, reason:
       'initPolymer does nothing with script tags where the program doesn\'t '
       'use a "package:" urls matching the same library.');
     expect(c, 2, reason: 'c was always imported with "package:" urls.');
     expect(d1.d, 1, reason: 'this matches the import from a');
     expect(d2.d, 1, reason: 'this matches the import from b');
   });
 }