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

VM developers rarely run the create_sdk build as it's significantly
slower than just building the VM. Since DartDev is coupled to the VM, we
should build it to ensure that there's no snapshot errors due to old
snapshots from previous create_sdk builds.

Change-Id: I7626e0f3c791f2a6e69830641d9abf043ec5138f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/144465
Reviewed-by: Jaime Wren <jwren@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>

utils/dartdev/BUILD.gn

@@ -18,8 +18,15 @@ dartfix_files = exec_script("../../tools/list_dart_files.py",
                             ],
                             "list lines")
 
-application_snapshot("dartdev") {
+copy("dartdev") {
+  deps = [ ":dartdev_snapshot" ]
+  sources = [ "$root_gen_dir/dartdev.dart.snapshot" ]
+  outputs = [ "$root_out_dir/dartdev.dart.snapshot" ]
+}
+
+application_snapshot("dartdev_snapshot") {
   main_dart = "../../pkg/dartdev/bin/dartdev.dart"
   training_args = [ "--help" ]
   inputs = dartdev_files + dartfix_files
+  output = "$root_gen_dir/dartdev.dart.snapshot"
 }