Fix created constructors for dart:html types

[jacob314]

Fix created constructors for dart:html types so that they stop generating uneccessary code that will probably mess up element rendering. The problem is we need to track that native fields on native classes do not need to be initialized. This is a P1 blocker for custom element support

For example the created constructor for InputElement looks like

    created() {
      this[dartx.accept] = null;
      this[dartx.alt] = null;
      this[dartx.autocapitalize] = null;
      this[dartx.autocomplete] = null;
      this[dartx.autofocus] = null;
      this[dartx.capture] = null;
      this[dartx.checked] = null;
      this[dartx.defaultChecked] = null;
      this[dartx.defaultValue] = null;
      this[dartx.dirName] = null;
      this[dartx.disabled] = null;
      this[dartx.files] = null;
      this[dartx.form] = null;
      this[dartx.formAction] = null;
      this[dartx.formEnctype] = null;
      this[dartx.formMethod] = null;
      this[dartx.formNoValidate] = null;
      this[dartx.formTarget] = null;
      this[dartx.height] = null;
      this[dartx.incremental] = null;
      this[dartx.indeterminate] = null;
      this[dartx.inputMode] = null;
      this[dartx.labels] = null;
      this[dartx.list] = null;
      this[dartx.max] = null;
      this[dartx.maxLength] = null;
      this[dartx.min] = null;
      this[dartx.minLength] = null;
      this[dartx.multiple] = null;
      this[dartx.name] = null;
      this[dartx.pattern] = null;
      this[dartx.placeholder] = null;
      this[dartx.readOnly] = null;
      this[dartx.required] = null;
      this[dartx.selectionDirection] = null;
      this[dartx.selectionEnd] = null;
      this[dartx.selectionStart] = null;
      this[dartx.size] = null;
      this[dartx.src] = null;
      this[dartx.step] = null;
      this[dartx.type] = null;
      this[dartx.validationMessage] = null;
      this[dartx.validity] = null;
      this[dartx.value] = null;
      this[_get_valueAsDate] = null;
      this[dartx.valueAsNumber] = null;
      this[dartx.entries] = null;
      this[dartx.directory] = null;
      this[dartx.width] = null;
      this[dartx.willValidate] = null;
      super.created();
    }

when it should look like

    created() {
      super.created();
    }

[jmesserly]

I thought custom elements were not P1? https://github.com/dart-lang/dev_compiler/issues/521