pkg/polymer[_expressions] unittest fixes

pkg/polymer/test/mirror_loader_4.dart

 library mirror_loader_test4;
 
-import 'dart:html';
 import 'package:polymer/polymer.dart';
 
 @CustomTag('x-a')
 class XA extends PolymerElement {
   final String x = "a";
   XA.created() : super.created();
 }
 
 // The next classes are here as a regression test for darbug.com/17929. Our
 // loader was incorrectly trying to retrieve the reflectiveType of these
 // classes.
 class Foo<T> {}
 
 @CustomTag('x-b')
 class XB<T> extends PolymerElement {
   final String x = "a";
   XB.created() : super.created();
 }