pkg/polymer[_expressions] unittest fixes

pkg/polymer/test/mirror_loader_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.
 
 import 'dart:async';
-import 'dart:html';
 
 import 'dart:mirrors';
 
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
-import 'package:path/path.dart' show url;
 
 import 'package:polymer/src/mirror_loader.dart';
 import 'mirror_loader_1.dart' as m1;
 import 'mirror_loader_2.dart' as m2;
 import 'mirror_loader_3.dart' as m3;
 import 'mirror_loader_4.dart' as m4;
 
 main() {
   useHtmlConfiguration();
 
@@ skipping 19 matching lines @@
       expect(e is UnsupportedError, isTrue);
       expect('$e', contains(
           'Custom element classes cannot have type-parameters: XB'));
     });
   });
 }
 
 final mirrors = currentMirrorSystem();
 _discover(Symbol name) =>
     discoverInitializers([mirrors.findLibrary(name).uri.toString()]);