anchor.diff:  /// APIs and the browser.
   abstract class CopyToClipboardStrategy {
     factory CopyToClipboardStrategy() {
  -    return (html.window.navigator.clipboard.writeText != null)
  +    return (html.window.navigator.clipboard?.writeText != null)
           ? ClipboardAPICopyStrategy()
           : ExecCommandCopyStrategy();
     }
  
positive.code: class ExecCommandCopyStrategy implements CopyToClipboardStrategy {
    @override
    void setData(String text) {
      // TODO(nurhan): https://github.com/flutter/flutter/issues/48578
      print('Clipboard is only implemented for browsers '
          'supporting Clipboard API. Use context menu for text editing.');
    }
  }
  
  
negative.code: abstract class PasteFromClipboardStrategy {
    factory PasteFromClipboardStrategy() {
      return (browserEngine == BrowserEngine.firefox ||
              html.window.navigator.clipboard?.readText == null)
          ? ExecCommandPasteStrategy()
          : ClipboardAPIPasteStrategy();
    }
  