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

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

Index: tools/dom/templates/html/dartium/html_dartium.darttemplate
diff --git a/tools/dom/templates/html/dartium/html_dartium.darttemplate b/tools/dom/templates/html/dartium/html_dartium.darttemplate
index 7cd522c372404a6faee30022be59761176f70e86..cb5e7dd2037f7cc3338f5fb6a261635e2a1e43df 100644
--- a/tools/dom/templates/html/dartium/html_dartium.darttemplate
+++ b/tools/dom/templates/html/dartium/html_dartium.darttemplate
@@ -399,8 +399,9 @@ wrap_jso(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.
@@ -429,7 +430,7 @@ wrap_jso(jsObject) {
       if (func != null) {
         dartClass_instance = func();
         dartClass_instance.blink_jsObject = jsObject;
-        jsObject.dartWrapper = dartClass_instance;
+        js.setDartHtmlWrapperFor(jsObject, dartClass_instance);
       }
     }
     return dartClass_instance;