Upgrade the JS HTMLElement to the Dart class. Used by Dart's Polymer.
Source
createCustomUpgrader(Type customElementClass, $this) {
var dartClass;
try {
dartClass = _blink.Blink_Utils.constructElement(customElementClass, $this);
} catch (e) {
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.
$this['dart_class'] = dartClass;
}
return dartClass;
}