Hide internal methods like wrap_jso from the dart:html public interface

tests/html/wrapping_collections_test.dart

 library wrapping_collection_test;
 
 import 'dart:html';
+import 'dart:html_common';
 import 'dart:js' as js;
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 
 /// Test that if we access objects through JS-interop we get the
 /// appropriate objects, even if dart:html maps them.
 main() {
   test("Access through JS-interop", () {
     var performance = js.context['performance'];
     var entries = performance.callMethod('getEntries', const []);
     entries.forEach((x) {
         expect(x is js.JsObject, isTrue);
     });
   });
 
   test("Access through dart:html", () {
     var dartPerformance = wrap_jso(js.context['performance']);
     var dartEntries = dartPerformance.getEntries();
     dartEntries.forEach((x) {
         expect(x is PerformanceEntry, isTrue);
     });
   });
 }