Report error only for elements we intend to upgrade.

pkg/web_components/lib/dart_support.js

 // Copyright (c) 2014, 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.
 
 // Teaches dart2js about the wrapping that is done by the Shadow DOM polyfill.
 (function() {
   var ShadowDOMPolyfill = window.ShadowDOMPolyfill;
   if (!ShadowDOMPolyfill) return;
 
   if (navigator.userAgent.indexOf('(Dart)') !== -1) {
@@ skipping 50 matching lines @@
         obj = unwrapped;
       }
       return originalGetTag(obj);
     }
   });
 })();
 
 // Updates document.registerElement so Dart can see when Javascript custom
 // elements are created, and wrap them to provide a Dart friendly API.
 (function (doc) {
-  var upgraders = {};
+  var upgraders = {};       // upgrader associated with a custom-tag.
+  var unpatchableTags = {}; // set of custom-tags that can't be patched.
   var originalRegisterElement = doc.registerElement;
   if (!originalRegisterElement) {
     throw new Error('document.registerElement is not present.');
   }
 
+  function reportError(name) {
+    console.error("Couldn't patch prototype to notify Dart when " + name +
+        " elements are created. This can be fixed by making the " +
+        "createdCallback in " + name + " a configurable property.");
+  }
+
   function registerElement(name, options) {
     var proto, extendsOption;
     if (options !== undefined) {
       proto = options.prototype;
     } else {
       proto = Object.create(HTMLElement.prototype);
       options = {protoptype: proto};
     }
 
     var original = proto.createdCallback;
     var newCallback = function() {
       original.call(this);
       var name = this.getAttribute('is') || this.localName;
       var upgrader = upgraders[name.toLowerCase()];
       if (upgrader) upgrader(this);
     };
 
     var descriptor = Object.getOwnPropertyDescriptor(proto, 'createdCallback');
     if (!descriptor || descriptor.writable) {
       proto.createdCallback = newCallback;
     } else if (descriptor.configurable) {
       descriptor['value'] = newCallback;
       Object.defineProperty(proto, 'createdCallback', descriptor);
     } else {
-      console.error("Couldn't patch prototype to notify Dart when " + name +
-          " elements are created. This can be fixed by making the " +
-          "createdCallback in " + name + " a configurable property.");
+      unpatchableTags[name] = true;
+      if (!!upgraders[name]) reportError(name);

[Jennifer Messerly, 2014/06/12 00:51:05]

I don't think !! is needed here.

[Siggi Cherem (dart-lang), 2014/06/12 01:03:23]

Done.

     }
     return originalRegisterElement.call(this, name, options);
   }
 
   function registerDartTypeUpgrader(name, upgrader) {
     if (!upgrader) return;
     name = name.toLowerCase();
     var existing = upgraders[name];
     if (existing) {
       console.error('Already have a Dart type associated with ' + name);
       return;
     }
     upgraders[name] = upgrader;
+    if (unpatchableTags[name]) reportError(name);
   }
 
 
   // Native custom elements outside the app in Chrome have constructor
   // names like "x-tag", which need to be translated to the DOM
   // element they extend.  When using the shadow dom polyfill this is
   // take care of above.
   var ShadowDOMPolyfill = window.ShadowDOMPolyfill;
   if (!ShadowDOMPolyfill) {
     // dartNativeDispatchHooksTransformer is described on initHooks() in
@@ skipping 12 matching lines @@
           }
           return originalGetUnknownTag(o, tag);
         }
       };
     });
   }
 
   doc._registerDartTypeUpgrader = registerDartTypeUpgrader;
   doc.registerElement = registerElement;
 })(document);