add factory for element wrappers

Fixes https://github.com/dart-lang/core-elements/issues/73
yeah, it's also a lot slower. you can use `new Element.tag('div', 'x-foo')` to replicate `<div is="x-foo">`
  
https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/dart-dom-html.Element#id_Element-tag
```dart
Element.tag(String tag, [String typeExtention])
```

document.createElement also supports the optional parameter: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/dart-dom-html.Document#id_createElement