Looks in default install locations on Mac, Linux and Windows for VS Code. If found, looks in default extension location to see if Dart Code is installed.
If VS Code is not installed, nothing is reported. If VS Code is installed without Dart Code, a warning is shown.
* Flatten change out from lots of merges and temporary hacks for #7224
* Expand fakes to cover cases where doctor passes
* -v != --verbose
* Cosmetic touchups
* delint
* Clean up summary line, add a switch for review comments
* Review comments
* nit
* review comment
* review comments