Return the real error during custom element created failure

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

 // Copyright (c) 2012, 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.
 
 // DO NOT EDIT
 // Auto-generated dart:html library.
 
 /**
  * HTML elements and other resources for web-based applications that need to
  * interact with the browser and the DOM (Document Object Model).
@@ skipping 387 matching lines @@
 /// ** Internal Use Only **
 @Deprecated("Internal Use Only")
 class DartHtmlDomObject {
 
   /// The underlying JS DOM object.
   @Deprecated("Internal Use Only")
   js.JsObject blink_jsObject;
 
 }
 
-/// Upgrade a Dart HtmlElement to the user's Dart custom element class.
-_upgradeHtmlElement(dartInstance) {
-  // Only try upgrading HtmlElement (Dart class) if there is a failure then
-  // don't try it again - one failure is enough.
-  if (dartInstance.runtimeType == HtmlElement && !dartInstance.isBadUpgrade) {
-    // Must be exactly HtmlElement not something derived from it.
-
-    var customElementClass = getCustomElementType(dartInstance);
-
-    // Custom Element to upgrade.
-    if (customElementClass != null) {
-      var jsObject = dartInstance.blink_jsObject;
-      try {
-        dartInstance = _blink.Blink_Utils.constructElement(customElementClass, jsObject);
-      } catch (e) {
-        dartInstance._badUpgrade();
-      } finally {
-        dartInstance.blink_jsObject = jsObject;
-        js.setDartHtmlWrapperFor(jsObject, dartInstance);
-     }
-   }
-  }
-
-  return dartInstance;
-}
-
 @Deprecated("Internal Use Only")
 class DebugAssertException implements Exception {
   String message;
   DebugAssertException(this.message);
 }
 
 @Deprecated("Internal Use Only")
 debug_or_assert(message, expression) {
   if (!expression) {
     throw new DebugAssertException("$message");
@@ skipping 29 matching lines @@
 }
 
 /**
  * Upgrade the JS HTMLElement to the Dart class.  Used by Dart's Polymer.
  */
 _createCustomUpgrader(Type customElementClass, $this) {
   var dartClass;
   try {
     dartClass = _blink.Blink_Utils.constructElement(customElementClass, $this);
   } catch (e) {
-    dartClass._badUpgrade();
+    // Did the dartClass get allocated but the created failed?  Otherwise, other
+    // components inside of this failed somewhere (could be JS custom element).
+    if (dartClass != null) {
+      // Yes, mark as didn't upgrade.
+      dartClass._badUpgrade();
+    }
     throw e;
   } finally {
     // Need to remember the Dart class that was created for this custom so
     // return it and setup the blink_jsObject to the $this that we'll be working
     // with as we talk to blink.
     js.setDartHtmlWrapperFor($this, dartClass);
   }
 
   return dartClass;
 }
 
 $else
 class DartHtmlDomObject extends NativeFieldWrapperClass2 {}
 
 _createCustomUpgrader(Type customElementClass, $this) => $this;
 
 $endif