• VSCode Version: 1.28.1
• OS Version: win10 1803

Steps to Reproduce:

1. I open a html file and click "Run Active File",
2. the editor alerts that "Cannot read property 'indexOf' of undefined" and nothing happened,
3. while in the last version it would open the html file in the browser correctly.
4. and I have uninstall vscode and delete the dir '.vscode/' and 'appdata/roaming/code/", then reinstall vscode, but it still not works.

Does this issue occur when all extensions are disabled?: Yes