visibilityState 属性

实现

@SupportedBrowser(SupportedBrowser.CHROME)
@SupportedBrowser(SupportedBrowser.FIREFOX)
@SupportedBrowser(SupportedBrowser.IE, '10')
String get visibilityState => JS(
    'String',
    '(#.visibilityState || #.mozVisibilityState || #.msVisibilityState ||'
        '#.webkitVisibilityState)',
    this,
    this,
    this,
    this);