Can't create a Polymer Core Element in dart - dart

Question

I basically want to create a <core-tooltip> tag, not in HTML, but in dart.
So i tried:
CoreTooltip tooltip = new CoreTooltip();
CoreTooltip tooltip = document.createElement("core-tooltip"):
CoreTooltip tooltip = new Element.tag("core-tooltip"):
got always the same Exception
Uncaught Error: type 'HtmlElement' is not a subtype of type 'CoreTooltip'
Why does that just not work?

Answer 1

You shouldn't use this method
document.createElement("core-tooltip"):
the other two are fine though.
I assume the element creation fails because the code is in a custom main and is executed before Polymer is done with initialization.
See how to implement a main function in polymer apps for more details.
If you execute the code inside a Polymer elements (for example attached() method after super.attached()) or in an event handler like on-click this will work.
Another possibility is, that you app is missing an HTML import that imports <core-tooltip>. Without an import this can't work either.
I tried it with this code and it worked for me
app_element.dart
import 'dart:html' as dom;
import 'package:polymer/polymer.dart';
import 'package:core_elements/core_tooltip.dart';
#CustomTag('app-element')
class AppElement extends PolymerElement {
AppElement.created() : super.created() { }
#PublishedProperty(reflect: true) bool isValidationError;
void attached() {
super.attached();
CoreTooltip tt = new CoreTooltip();
print(tt);
}
}
app_element.html
<link rel="import" href="../../packages/polymer/polymer.html">
<link rel="import" href="../../packages/core_elements/core_tooltip.html">
<polymer-element name="app-element">
<template>
</template>
<script type="application/dart" src="app_element.dart"></script>
</polymer-element>