[flutter_plugin_tools] Fix federated safety check (#4368)

The new safety check doesn't allow simple platform-interface-only
changes because it doesn't actually check that a non-interface package
is actually modified before failing it for a modified platform
interface.

This fixes that, and adds a test case covering it.

script/tool/lib/src/federation_safety_check_command.dart

@@ -135,6 +135,13 @@ class FederationSafetyCheckCommand extends PackageLoopingCommand {
       return PackageResult.success();
     }
 
+    final List<String> changedPackageFiles =
+        _changedDartFiles[package.directory.basename] ?? <String>[];
+    if (changedPackageFiles.isEmpty) {
+      print('No Dart changes.');
+      return PackageResult.success();
+    }
+
     // If the change would be flagged, but it appears to be a mass change
     // rather than a plugin-specific change, allow it with a warning.
     //