[build] Use an absolute path for the snapshot.

When constructing the command line for creating the kernel service
snapshot, the variable `abs_output` actually contains a relative
path, which means that copy/pasting the generated command line fails
if not within the root build directory. This CL fixes that.

Change-Id: I0d9fcafac489a6bd4771de1d6db1a2aa3623dcf8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/200861
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Tess Strickland <sstrickl@google.com>
diff --git a/utils/application_snapshot.gni b/utils/application_snapshot.gni
index 7ecdf28..51d2b147 100644
--- a/utils/application_snapshot.gni
+++ b/utils/application_snapshot.gni
@@ -145,7 +145,7 @@
     dfe = "$_dart_root/pkg/vm/bin/kernel_service.dart"
 
     abs_depfile = rebase_path(depfile)
-    abs_output = rebase_path(output, root_build_dir)
+    abs_output = rebase_path(output)
 
     vm_args = [
                 "--deterministic",