Update to handle expections in custom element's created constructor

tools/dom/templates/html/impl/impl_HTMLDocument.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.
 
 // WARNING: Do not edit - generated code.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
 $!MEMBERS
@@ skipping 292 matching lines @@
     //
     //              sanitizing causes custom element to created recursively
     //              until stack overflow.
     //
     //              See https://github.com/dart-lang/sdk/issues/23666
     int creating = 0;
     elemProto['createdCallback'] = new js.JsFunction.withThis(($this) {
       if (_getJSClassName(reflectClass(customElementClass).superclass) != null && creating < 2) {
         creating++;
 
-        var dartClass = _blink.Blink_Utils.constructElement(customElementClass, $this);
+        var dartClass;
+        try {
+          dartClass = _blink.Blink_Utils.constructElement(customElementClass, $this);
+        } catch (e) {
+          dartClass = null;
+        } 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. 
+          $this['dart_class'] = dartClass;
 
-        // 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. 
-        $this['dart_class'] = dartClass;
-
-        creating--;
+          creating--;
+        }
       }
     });
     elemProto['attributeChangedCallback'] = new js.JsFunction.withThis(($this, attrName, oldVal, newVal) {
       if ($this["dart_class"] != null && $this['dart_class'].attributeChanged != null) {
         $this['dart_class'].attributeChanged(attrName, oldVal, newVal);
       }
     });
     elemProto['attachedCallback'] = new js.JsFunction.withThis(($this) {
       if ($this["dart_class"] != null && $this['dart_class'].attached != null) {
         $this['dart_class'].attached();
@@ skipping 67 matching lines @@
   /// parameter must be provided.
   @Experimental()
   ElementUpgrader createElementUpgrader(Type type, {String extendsTag}) {
 $if DART2JS
     return new _JSElementUpgrader(this, type, extendsTag);
 $else
     return new _VMElementUpgrader(this, type, extendsTag);
 $endif
   }
 }