title: VSCode + Flutter + Dart => Rename Symbol is really slow and sometimes not even working
body: <p>Rename symbol (shortcut key F2) in my flutter projects is really slow. Not sure if flutter is relevant of if this is a general dart problem. When I have syntax/build errors I find it common that rename never finishes, the "infinite progress" in VS Code just keeps spinning what seems to be forever.</p>
  
  <p>If I create a brand new project: <code>flutter create slow_refactor</code> and then open the project, let everything initialize, pub get packages and then let it complete. After this I go to main.dart, rename <code>title</code> to <code>myTitle</code>. This takes ... 10? 15? seconds to complete. If I rename it to <code>myTitle2</code> it takes a split second.</p>
  
  <p>Can I mitigate this issue somehow? It's really tedious to rename symbols and most often I just hit replace all feature instead because I'm convinced that the rename won't work. Rename is such a common (simple) refactor that I expect it to work at all times.</p>
  
  <p>I can't find any known issues regarding this but I know that other members in my team have the same issue.</p>
  
  <p>I'm running Windows 10, stable VS Code, flutter 1.13 and 1.14 (beta and master), dart 2.8.0-dev.3.0. My computer specs are category "great" so this is not due to slow cpu, disk or memory issues.</p>