[dtd] Update the dtd analysis_options.yaml to match the settings under dtd_impl

Change-Id: I35ce2fc2b96181fb82349d2f5589a25c8188e2c5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/488901
Reviewed-by: Ben Konyi <bkonyi@google.com>
Auto-Submit: Jaime Wren <jwren@google.com>
Commit-Queue: Jaime Wren <jwren@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>

pkg/dtd/analysis_options.yaml

@@ -1,10 +1,13 @@
 include: package:dart_flutter_team_lints/analysis_options.yaml
 
-analyzer:
-  language:
-    strict-raw-types: true
-
 linter:
   rules:
     - avoid_void_async
+    - unawaited_futures
     - require_trailing_commas
+    - prefer_relative_imports
+
+analyzer:
+  language:
+    strict-inference: true
+    strict-raw-types: true