Reland "[ DartDev ] Generate dartdev.dart.snapshot for runtime builds and place it in the root of the build output directory"

This reverts commit 60823bdaf6.

Change-Id: I10a8421cb1fc5738bfb56d3c4e68d05bb3b4edd9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/144680
Reviewed-by: Jaime Wren <jwren@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>

utils/dartdev/BUILD.gn

@@ -18,8 +18,20 @@ dartfix_files = exec_script("../../tools/list_dart_files.py",
                             ],
                             "list lines")
 
-application_snapshot("dartdev") {
+group("dartdev") {
+  deps = [ ":copy_dartdev_snapshot" ]
+}
+
+copy("copy_dartdev_snapshot") {
+  visibility = [ ":dartdev" ]
+  deps = [ ":generate_dartdev_snapshot" ]
+  sources = [ "$root_gen_dir/dartdev.dart.snapshot" ]
+  outputs = [ "$root_out_dir/dartdev.dart.snapshot" ]
+}
+
+application_snapshot("generate_dartdev_snapshot") {
   main_dart = "../../pkg/dartdev/bin/dartdev.dart"
   training_args = [ "--help" ]
   inputs = dartdev_files + dartfix_files
+  output = "$root_gen_dir/dartdev.dart.snapshot"
 }