[flutter] Update built value to 7.0.0

Change-Id: Iabd2474e7dcdf1d0dd632de3ae05083cf4506354
diff --git a/tools/dart_dependencies.yaml b/tools/dart_dependencies.yaml
index ea9e83f..42508f7 100644
--- a/tools/dart_dependencies.yaml
+++ b/tools/dart_dependencies.yaml
@@ -5,7 +5,7 @@
 name: modules
 dependencies:
   built_collection: ^4.0.0
-  built_value: ^6.1.4
+  built_value: '>=6.1.4 <8.0.0'
   cli_util: ^0.1.0
   fake_async: ^1.0.1
   flutter_flux: ^4.1.2