Audit the CONTRIBUTING.md instructions for windows users

[kenzieschmoll]

We should walk through this workflow for Windows and make necessary adjustments for things that are inaccurate or missing.
https://github.com/flutter/devtools/blob/master/CONTRIBUTING.md

[DanTup]

A few notes (some not necessarily Windows-specific):

• Change your local Flutter SDK to the latest flutter candidate branch: ./tool/update_flutter_sdk.sh --local
  This won't currently work for Windows users - fixed by Rewrite update_flutter_sdk.sh as a Dart script #6604
• "...and the "Chrome" device"
  Could be more explicit how/where to set the device
• export LOCAL_DART_SDK=/path/to/dart/sdk
  This needs to be set in the Windows Environment Variables dialog (similar to PATH) for Windows users
• build_e2e.dart error when LOCAL_DART_SDK is not set is also slightly misleading for Windows users
• build_e2e.dart might not work on Windows (it calls ./tool/build_release.sh)
• Running DevTools on Flutter Desktop
  Is there a reason this only gives a CLI command and doesn't note this can be done in VS Code by just changing the device?
• Enable and activate DCM**
  This mentions external contributors might not get all DCM rules but it's now free for OSS projects - I wonder if this covers DevTools so external contributors would get the full version?

Some of these are simple text tweaks, though probably update_flutter_sdk.sh is the most significant (which relies on latest_flutter_candidate, although I wonder if it could just read from the flutter-candidate.txt file?).

[kenzieschmoll]

Some of these are simple text tweaks, though probably update_flutter_sdk.sh is the most significant (which relies on latest_flutter_candidate, although I wonder if it could just read from the flutter-candidate.txt file?).

Great idea on reading from flutter-candidate.txt. #6604 implements this change.

[FMorschel]

I know this is not Windows-specific, but even though running dart run build_runner build is known to most devs, we should still add this instruction for new devs that may never have encountered this. If you think I should open a new issue for this only please let me know.

Edit: If I remember correctly this also needs to be enabled globally or something? So probably that too.

[DanTup]

I reviewed this doc again and checked against the list of items above. It appears that they've all been addressed or are no longer relevant (for example many of the commands were rolled into the dt tool).

@FMorschel I'm not sure I understand your comment, I don't think you should need to manually run dart run build_runner build, just use the dt tool?