anchor.diff:  @NgDirective(selector: '[ng-required]', map: const {'ng-required': '=>required'})
   @NgDirective(selector: '[ng-selected]', map: const {'ng-selected': '=>selected'})
   class NgBooleanAttribute {
  -  final NodeAttrs attrs;
  -  NgBooleanAttribute(this.attrs);
  +  final NgElement _ngElement;
   
  -  _setBooleanAttribute(name, value) => attrs[name] = (toBool(value) ? '' : null);
  +  NgBooleanAttribute(this._ngElement);
   
  -  set checked(value)   => _setBooleanAttribute('checked',  value);
  -  set disabled(value)  => _setBooleanAttribute('disabled', value);
  -  set multiple(value)  => _setBooleanAttribute('multiple', value);
  -  set open(value)      => _setBooleanAttribute('open',     value);
  -  set readonly(value)  => _setBooleanAttribute('readonly', value);
  -  set required(value)  => _setBooleanAttribute('required', value);
  -  set selected(value)  => _setBooleanAttribute('selected', value);
  +  void set checked(on)  => _toggleAttribute('checked',  on);
  +  void set disabled(on) => _toggleAttribute('disabled', on);
  +  void set multiple(on) => _toggleAttribute('multiple', on);
  +  void set open(on)     => _toggleAttribute('open', on);
  +  void set readonly(on) => _toggleAttribute('readonly', on);
  +  void set required(on) => _toggleAttribute('required', on);
  +  void set selected(on) => _toggleAttribute('selected', on);
  +
  +  void _toggleAttribute(attrName, on) {
  +    if (toBool(on)) {
  +      _ngElement.setAttribute(attrName);
  +    } else {
  +      _ngElement.removeAttribute(attrName);
  +    }
  +  }
   }
   
   /**
  
positive.code:  *
   * Using `<button disabled="{{false}}">` does not work since it would result
   * in `<button disabled="false">` rather than `<button>`.
   * Browsers change behavior based on presence/absence of attribute rather the
   * its value.
   *
   * For this reason we provide alternate `ng-`attribute directives to
  
negative.code: @NgDirective(selector: '[ng-href]',   map: const {'ng-href':   '@href'})
  @NgDirective(selector: '[ng-src]',    map: const {'ng-src':    '@src'})
  @NgDirective(selector: '[ng-srcset]', map: const {'ng-srcset': '@srcset'})
  class NgSource {
    final NgElement _ngElement;
    NgSource(this._ngElement);
  
    void set href(value)   => _ngElement.setAttribute('href', value);
    void set src(value)    => _ngElement.setAttribute('src', value);
    void set srcset(value) => _ngElement.setAttribute('srcset', value);
  
  }
  
  