If first invocation of a Dart script, as a @tool, crashes, the snapshot is not made, but dartdoc does not stop.

[srawlins]

This was discovered testing 0.33.0 on flutter.

[srawlins]

I am hitting this today again. Snapshotting is failing because of some issue with null-safety. When I run it manually:

$ /Users/srawlins/code/flutter/bin/cache/dart-sdk/bin/dart --ignore-unrecognized-flags --verbosity=error --snapshot=/var/folders/84/mf667nnx3k72cb2h2b2btt6r005w15
/T/dartdoc_snapshot_cache_yEVyeM/snapshot_0 --snapshot_kind=app-jit /Users/srawlins/code/flutter/dev/snippets/lib/main.dart --type=sample --template=stateless_widget                              
../snippets/lib/configuration.dart:44:12: Error: Null safety features are disabled for this library.                                                                                               
Try removing the package language version or setting the language version to 2.12 or higher.                                                                                                       
  Directory? _configPath;                                                                                                                                                                          
           ^

[srawlins]

The two issues I see here are:

• the toolErrorCallback does not actually print any warning to the terminal, so all following errors are a mystery.
• the tool will continue to be run; if the snapshot failed, the tool should not be run any more.

[srawlins]

Small note: the debugging print I put here: #2690 does print, but the warning won't show, on a dart invocation which fails to produce a snapshot.