Update pub to use the VM's new bad-snapshot exit code.

sdk/lib/_internal/pub/lib/src/executable.dart

Index: sdk/lib/_internal/pub/lib/src/executable.dart
diff --git a/sdk/lib/_internal/pub/lib/src/executable.dart b/sdk/lib/_internal/pub/lib/src/executable.dart
index 966b68f6d7f6efb6b980c407cc3d531ad787f646..1c22b801339ed241c6455c2a2d57672bd501253c 100644
--- a/sdk/lib/_internal/pub/lib/src/executable.dart
+++ b/sdk/lib/_internal/pub/lib/src/executable.dart
@@ -186,9 +186,9 @@ Future<int> runSnapshot(String path, Iterable<String> args, {recompile(),
   }
 
   var exitCode = await runProcess(stdin1);
-  if (recompile == null || exitCode != 255) return exitCode;
+  if (recompile == null || exitCode != 253) return exitCode;
 
-  // Exit code 255 indicates that the snapshot version was out-of-date. If we
+  // Exit code 253 indicates that the snapshot version was out-of-date. If we
   // can recompile, do so.
   await recompile();
   return runProcess(stdin2);