Cache all wrap_jso results so we preserve identity

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..da243101c720fddc74e2ea0293c7ae0b370a006c 100644
--- a/tools/dom/templates/html/dartium/html_dartium.darttemplate
+++ b/tools/dom/templates/html/dartium/html_dartium.darttemplate
@@ -394,6 +394,12 @@ wrap_jso(jsObject) {
       // JS Interop converted the object to a Dart class e.g., Uint8ClampedList.
       return jsObject;
     }
+
+    // To preserve identity, if we already have a wrapper for this, return it.
+    var existing = jsObject['dartClass_instance'];
+    if (existing != null) {
+      return existing;
+    }
     // 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 +426,7 @@ wrap_jso(jsObject) {
       if (func != null) {
         dartClass_instance = func();
         dartClass_instance.blink_jsObject = jsObject;
+        jsObject['dartClass_instance'] = dartClass_instance;
       }
     }
     return dartClass_instance;
@@ -435,6 +442,7 @@ wrap_jso(jsObject) {
   return null;
 }
 
+
 /**
  * Create Dart class that maps to the JS Type that is the JS type being
  * extended using JS interop createCallback (we need the base type of the