add support for the builders configuration for web (#50988)

packages/flutter_tools/lib/src/codegen.dart

@@ -194,7 +194,9 @@ class CodeGeneratingResidentCompiler implements ResidentCompiler {
   }
 
   @override
-  void addFileSystemRoot(String root) { }
+  void addFileSystemRoot(String root) {
+    _residentCompiler.addFileSystemRoot(root);
+  }
 }
 
 /// The current status of a codegen build.

packages/flutter_tools/lib/src/resident_runner.dart

@@ -110,13 +110,15 @@ class FlutterDevice {
         experimentalFlags: experimentalFlags,
         dartDefines: dartDefines,
       );
+    }
+
     if (flutterProject.hasBuilders) {
       generator = await CodeGeneratingResidentCompiler.create(
         residentCompiler: generator,
         flutterProject: flutterProject,
       );
     }
-    }
+
     return FlutterDevice(
       device,
       trackWidgetCreation: trackWidgetCreation,