PubToplevel.directory is never null. (#3587)

bin/dependency_services.dart

diff --git a/bin/dependency_services.dart b/bin/dependency_services.dart
index f117afd..90e528a 100644
--- a/bin/dependency_services.dart
+++ b/bin/dependency_services.dart
@@ -20,7 +20,7 @@
 class _DependencyServicesCommandRunner extends CommandRunner<int>
     implements PubTopLevel {
   @override
-  String? get directory => argResults['directory'];
+  String get directory => argResults['directory'];
 
   @override
   bool get captureStackChains => argResults['verbose'];

lib/src/command.dart

diff --git a/lib/src/command.dart b/lib/src/command.dart
index 4a65fb1..31f0b17 100644
--- a/lib/src/command.dart
+++ b/lib/src/command.dart
@@ -55,11 +55,12 @@
     return a;
   }
 
-  String get directory =>
-      (argResults.options.contains('directory')
-          ? argResults['directory']
-          : null) ??
-      _pubTopLevel.directory;
+  String get directory {
+    return (argResults.options.contains('directory')
+            ? argResults['directory']
+            : null) ??
+        _pubTopLevel.directory;
+  }
 
   late final SystemCache cache = SystemCache(isOffline: isOffline);
 
@@ -350,7 +351,7 @@
   }
 
   /// The directory containing the pubspec.yaml of the project to work on.
-  String? get directory;
+  String get directory;
 
   /// The argResults from the level of parsing of the 'pub' command.
   ArgResults get argResults;

lib/src/command_runner.dart

diff --git a/lib/src/command_runner.dart b/lib/src/command_runner.dart
index 92d4cf8..150de16 100644
--- a/lib/src/command_runner.dart
+++ b/lib/src/command_runner.dart
@@ -45,7 +45,7 @@
 
 class PubCommandRunner extends CommandRunner<int> implements PubTopLevel {
   @override
-  String? get directory => argResults['directory'];
+  String get directory => argResults['directory'];
 
   @override
   bool get captureStackChains {