[dds/dap] Address code review nits

Change-Id: I7a03650ba1d8df7b0f11db3408f9e55525b61aa9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/266580
Commit-Queue: Ben Konyi <bkonyi@google.com>
Reviewed-by: Ben Konyi <bkonyi@google.com>

pkg/dds/lib/src/dap/adapters/dart.dart

@@ -332,7 +332,7 @@ abstract class DartDebugAdapter<TL extends LaunchRequestArguments,
   ///
   /// Keys are the base file paths and the values are the base URIs. Neither
   /// value should contain trailing slashes.
-  final Map<String, Uri> orgDartlangSdkMappings = {};
+  final orgDartlangSdkMappings = <String, Uri>{};
 
   /// The DDS instance that was started and that [vmService] is connected to.
   ///