In the buildbot CI step, reset built web files before trying to publish (dart-lang#3896)

These files can have minor differences in them (depending on the version of
Dart?), so that it is difficult for them to always be identical to the ones
checked in, during CI. Then there is a new warning that `pub publish --dry-run`
produces, if the current git checkout is not clean:
dart-lang/pub#4373. We need to reset these files to
avoid that warning.

Author: srawlins

tool/task.dart

@@ -834,6 +834,15 @@ Rebuild them with "dart tool/task.dart build" and check the results in.
         'The web frontend (web/docs.dart) needs to be recompiled; rebuild it '
         'with "dart tool/task.dart build web".');
   }
+
+  // Reset some files for `try-publish` step. This check looks for changes in
+  // the current git checkout: https://github.com/dart-lang/pub/pull/4373.
+  Process.runSync('git', [
+    'checkout',
+    '--',
+    'lib/resources/docs.dart.js',
+    'lib/resources/docs.dart.js.map',
+  ]);
 }
 
 /// Whether the analyzer in use (as found in `pubspec.lock`) is the target