Preserve identity of Dart wrappers on DOM objects

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 b15dab237afe597501927d563a74b151841e4748..7cd522c372404a6faee30022be59761176f70e86 100644
--- a/tools/dom/templates/html/dartium/html_dartium.darttemplate
+++ b/tools/dom/templates/html/dartium/html_dartium.darttemplate
@@ -390,10 +390,19 @@ unwrap_jso(dartClass_instance) {
  */
 wrap_jso(jsObject) {
   try {
-    if (jsObject is! js.JsObject) {
+    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;
+    }
+
     // 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
@@ -420,6 +429,7 @@ wrap_jso(jsObject) {
       if (func != null) {
         dartClass_instance = func();
         dartClass_instance.blink_jsObject = jsObject;
+        jsObject.dartWrapper = dartClass_instance;
       }
     }
     return dartClass_instance;