[build] Make app snapshot build failures copy/pastable.

* Use a filename without spaces for the non-existent DFE path.

* Convert a relative path to the executable to an absolute one in
  gn_run_binary so the command can be run outside the build directory
  without editing.

Change-Id: I31421acb54ad940115d4e000c4ec31605a33b5cf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/216800
Reviewed-by: Alexander Thomas <athom@google.com>

build/gn_run_binary.py

@@ -45,12 +45,9 @@ def main(argv):
         return 1
 
     # Unless the path is absolute, this script is designed to run binaries
-    # produced by the current build. We always prefix it with "./" to avoid
-    # picking up system versions that might also be on the path.
-    if os.path.isabs(argv[2]):
-        path = argv[2]
-    else:
-        path = './' + argv[2]
+    # produced by the current build, which is the current working directory when
+    # this script is run.
+    path = os.path.abspath(argv[2])
 
     if not os.path.isfile(path):
         print("Binary not found: " + path)

utils/application_snapshot.gni

@@ -148,7 +148,7 @@ template("_application_snapshot") {
 
     # Explicitly set DFE so Dart doesn't implicitly depend on the kernel service
     # snapshot (creating a circular dep. for kernel-service_snapshot).
-    dfe = "NEVER LOADED"
+    dfe = "NEVER_LOADED"
 
     abs_depfile = rebase_path(depfile)
     abs_output = rebase_path(output)