Delete stale compiled files.

sdk/lib/_internal/pub_generated/lib/src/barback/asset_environment.dart

 library pub.barback.asset_environment;
 import 'dart:async';
 import 'dart:io';
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as path;
 import 'package:watcher/watcher.dart';
 import '../entrypoint.dart';
 import '../exceptions.dart';
 import '../io.dart';
 import '../log.dart' as log;
@@ skipping 114 matching lines @@
         var basename = path.url.basename(id.path);
         var snapshotPath = path.join(directory, "$basename.snapshot");
         return runProcess(
             Platform.executable,
             [
                 '--snapshot=$snapshotPath',
                 server.url.resolve(basename).toString()]).then((result) {
           if (result.success) {
             log.message("Precompiled ${_formatExecutable(id)}.");
           } else {
-            deleteEntry(snapshotPath);
             throw new ApplicationException(
                 log.yellow("Failed to precompile " "${_formatExecutable(id)}:\n") +
                     result.stderr.join('\n'));
           }
         });
       })).whenComplete(() {
         server.close();
       });
     });
   }
@@ skipping 305 matching lines @@
   const _PollingWatcherType();
   DirectoryWatcher create(String directory) =>
       new PollingDirectoryWatcher(directory);
   String toString() => "polling";
 }
 class _NoneWatcherType implements WatcherType {
   const _NoneWatcherType();
   DirectoryWatcher create(String directory) => null;
   String toString() => "none";
 }