Use allowed_experiments.json when building SDK summary.

Change-Id: I3fd73d9890d62cc7b3c19e061069979a6b65c9b7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/149052
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

utils/dartanalyzer/BUILD.gn

@@ -46,13 +46,13 @@ prebuilt_dart_action("generate_summary_strong") {
   outputs = [ output ]
   if (!dont_use_nnbd) {
     args = [
-      "build-non-nullable",
+      "build-strong",
       rebase_path(output),
       rebase_path("../../sdk_nnbd"),
     ]
   } else {
     args = [
-      "build-legacy",
+      "build-strong",
       rebase_path(output),
       rebase_path("../../sdk"),
     ]