Bump built_value from 8.3.2 to 8.4.0

Bumps [built_value](https://github.com/google/built_value.dart) from 8.3.2 to 8.4.0.
- [Release notes](https://github.com/google/built_value.dart/releases)
- [Changelog](https://github.com/google/built_value.dart/blob/master/CHANGELOG.md)
- [Commits](google/built_value.dart@v8.3.2...v8.4.0)

---
updated-dependencies:
- dependency-name: built_value
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

pubspec.lock

@@ -91,14 +91,14 @@ packages:
       name: built_value
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "8.3.2"
+    version: "8.4.0"
   built_value_generator:
     dependency: "direct dev"
     description:
       name: built_value_generator
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "8.3.2"
+    version: "8.4.0"
   characters:
     dependency: transitive
     description:

pubspec.yaml

@@ -17,7 +17,7 @@ dependencies:
   meta: ^1.7.0
   path: ^1.8.1
   http: ^0.13.5
-  built_value: ^8.3.2
+  built_value: ^8.4.0
   tuple: ^2.0.0
   rxdart: ^0.27.5
   image_picker: ^0.8.5+3