Use prebuilt dart to generate strong sdk summary

This saves ~5 minutes on Flutter Engine linux arm64 cross-builds.

Change-Id: I67d849422886101c217de7c2f21bc4eba4f6706a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/201042
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Zach Anderson <zra@google.com>
diff --git a/utils/dartanalyzer/BUILD.gn b/utils/dartanalyzer/BUILD.gn
index 0c007ab..d079d06 100644
--- a/utils/dartanalyzer/BUILD.gn
+++ b/utils/dartanalyzer/BUILD.gn
@@ -32,7 +32,7 @@
   output = "$target_gen_dir/sdk_lib_files.stamp"
 }
 
-dart_action("generate_summary_strong") {
+prebuilt_dart_action("generate_summary_strong") {
   deps = [
     ":sdk_lib_files",
     "../../sdk:copy_libraries",