Make the dartWrapper access on JsObject private and provide top-level functions for it

tools/dom/templates/html/dartium/html_dartium.darttemplate

 // Copyright (c) 2012, 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.
 
 // DO NOT EDIT
 // Auto-generated dart:html library.
 
 /**
  * HTML elements and other resources for web-based applications that need to
  * interact with the browser and the DOM (Document Object Model).
@@ skipping 381 matching lines @@
   try {
     if (jsObject is! js.JsObject || jsObject == null) {
       // JS Interop converted the object to a Dart class e.g., Uint8ClampedList.
       // or it's a simple type.
       return jsObject;
     }
 
     // TODO(alanknight): With upgraded custom elements this causes a failure because
     // we need a new wrapper after the type changes. We could possibly invalidate this
     // if the constructor name didn't match?
-    if (jsObject.dartWrapper != null) {
-      return jsObject.dartWrapper;
+    var wrapper = js.getDartHtmlWrapperFor(jsObject);
+    if (wrapper != null) {
+      return wrapper;
     }
 
     // Try the most general type conversions on it.
     // TODO(alanknight): We may be able to do better. This maintains identity,
     // which is useful, but expensive. And if we nest something that only
     // this conversion handles, how does that work? e.g. a list of maps of elements.
     var converted = convertNativeToDart_SerializedScriptValue(jsObject);
     if (!identical(converted, jsObject)) {
       return converted;
     }
     var constructor = jsObject['constructor'];
     if (__interop_checks) {
       debug_or_assert("constructor != null", constructor != null);
     }
     var jsTypeName = constructor['name'];
     if (__interop_checks) {
       debug_or_assert("constructor != null && jsTypeName.length > 0", constructor != null && jsTypeName.length > 0);
     }
 
     var dartClass_instance;
     if (jsObject.hasProperty('dart_class')) {
       // Got a dart_class (it's a custom element) use it it's already set up.
       dartClass_instance = jsObject['dart_class'];
     } else {
       var func = getHtmlCreateFunction(jsTypeName);
       if (func != null) {
         dartClass_instance = func();
         dartClass_instance.blink_jsObject = jsObject;
-        jsObject.dartWrapper = dartClass_instance;
+        js.setDartHtmlWrapperFor(jsObject, dartClass_instance);
       }
     }
     return dartClass_instance;
   } catch(e, stacktrace){
     if (__interop_checks) {
       if (e is DebugAssertException)
         window.console.log("${e.message}\n ${stacktrace}");
       else
         window.console.log("${stacktrace}");
     }
@@ skipping 109 matching lines @@
 class JsoNativeFieldWrapper extends NativeFieldWrapperClass2 {}
 
 unwrap_jso(dartClass_instance) => dartClass_instance;
 wrap_jso(jsObject) => jsObject;
 make_dart_rectangle(r) => r;
 convertDartToNative_Dictionary(Map dict) => dict;
 List convertDartToNative_StringArray(List<String> input) => input;
 convertDartToNative_List(List input) => input;
 
 $endif