Dart SDK Repo is no longer detected correctly #3933

We usually detect when you're in the Dart SDK repo and disable any "pub get" prompts or automatic fetching of packages. This check was done by looking for:

• README.dart-sdk
• .dart_tool/package_config.json

Since dart-lang/sdk@47c011a, the second file no longer exists. We need to tweak this check (or figure out something more generic for non-Pub workspaces).