Report error only for elements we intend to upgrade.

pkg/web_components/lib/dart_support.js

Index: pkg/web_components/lib/dart_support.js
diff --git a/pkg/web_components/lib/dart_support.js b/pkg/web_components/lib/dart_support.js
index 77948eb29438f24008c56db0078bc50a54249610..895adcc92803b21b3e9bdd69df710d0ee8c1ed98 100644
--- a/pkg/web_components/lib/dart_support.js
+++ b/pkg/web_components/lib/dart_support.js
@@ -68,12 +68,19 @@
 // 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) {
@@ -98,9 +105,8 @@
       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);
   }
@@ -114,6 +120,7 @@
       return;
     }
     upgraders[name] = upgrader;
+    if (unpatchableTags[name]) reportError(name);
   }