[ddc] Use sound null safety when training snapshot

Change-Id: I89761a8248fbca64ed066dafbc3c5ece8be1c455
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/291302
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>

utils/dartdevc/BUILD.gn

@@ -26,8 +26,7 @@ application_snapshot("dartdevc") {
 
   training_args = [
     "--dart-sdk-summary",
-    rebase_path(sdk_outline_unsound_dill),
-    "--no-sound-null-safety",
+    rebase_path(sdk_outline_sound_dill),
     "-o",
     "dartdevc.js",
     rebase_path("../../pkg/dev_compiler/bin/dartdevc.dart"),
@@ -37,12 +36,8 @@ application_snapshot("dartdevc") {
     training_args += [ "--canary" ]
   }
 
-  deps = [
-    ":dartdevc_kernel_sdk",
-    ":dartdevc_platform_unsound",
-  ]
-
-  inputs = [ sdk_outline_unsound_dill ]
+  deps = [ ":dartdevc_platform_sound" ]
+  inputs = [ sdk_outline_sound_dill ]
 }
 
 sdk_root = "../../sdk"