Bump meta package to 1.3.0-nullsafety.4

Change-Id: I693e4aee8c14177e3a72ef421b96637e4b1d2037
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/166340
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

Author: srawlins

pkg/meta/pubspec.yaml

@@ -1,5 +1,5 @@
 name: meta
-version: 1.3.0-nullsafety.3
+version: 1.3.0-nullsafety.4
 homepage: https://github.com/dart-lang/sdk/tree/master/pkg/meta
 description: >
  This library contains the declarations of annotations that developers can use