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

sdk/lib/html/html_common/conversions_dart2js.dart

Index: sdk/lib/html/html_common/conversions_dart2js.dart
diff --git a/sdk/lib/html/html_common/conversions_dart2js.dart b/sdk/lib/html/html_common/conversions_dart2js.dart
index ac5b7554de34e5956e2ec596a4299dd9d1850916..3b3a8adf3960f50b75bda109d7907acb1a2bd577 100644
--- a/sdk/lib/html/html_common/conversions_dart2js.dart
+++ b/sdk/lib/html/html_common/conversions_dart2js.dart
@@ -91,3 +91,9 @@ Future convertNativePromiseToDartFuture(promise) {
   var newPromise = JS('', '#.then(#).catch(#)', promise, then, error);
   return completer.future;
 }
+
+/// Wrap a JS object with an instance of the matching dart:html class. Used only in Dartium.
+wrap_jso(jsObject) => jsObject;
+
+/// Find the underlying JS object for a dart:html Dart object.
+unwrap_jso(dartClass_instance) => dartClass_instance;