adding clipboard support for git clone dialog

@gaurave PR for https://github.com/dart-lang/spark/issues/2235 


Reason for this is copy/paste from `document.execCommand` requires that input boxes be displayed, otherwise they silently fail. So by putting the call in `_invoke` will require the `_copyClipboard()` to be called from a `Timer.run` callback after `_show();`

```
   void _invoke([Object context]) {
      // Select any previous text in the URL field.
      Timer.run(_repoUrlElement.select);
      // Show folder picker, if top-level folder is not set.
      spark.projectLocationManager.getProjectLocation().then((LocationResult r) {
        if (r != null) {
          _show();
         // Has to be in Timer.run since _show wraps _dialog.show() in a Timer.run also.
         Timer.run(_copyClipboard);
        }
      });
    }
```

Just wanted to explain why, still want me to use the inline `Timer.run()` after `_show();` call?