Debugging through SDK files creates lots of duplicates in the open file list

[DanTup]

Happened while debugging pub_package_service_test.dart (might have been across multiple sessions).

[DanTup]

Doesn't seem like there's much we can do here, so have raised with VS Code: microsoft/vscode#131502

[DanTup]

This is resolved by the new SDK DAPs because we don't download sources anymore. This currently requires the dart.previewSdkDaps setting be enabled, but over the coming releases this will gradually become the default for everyone on new SDKs.