[flutter_tool] Make the registrant import relative to the entrypoint (#50302)

packages/flutter_tools/lib/src/build_runner/build_script.dart

@@ -351,13 +351,14 @@ class FlutterWebShellBuilder implements Builder {
       return;
     }
     final AssetId outputId = buildStep.inputId.changeExtension('_web_entrypoint.dart');
+    final String pluginRegistrantPath = _getPluginRegistrantPath(dartEntrypointId.path);
     if (hasPlugins) {
       await buildStep.writeAsString(outputId, '''
 import 'dart:ui' as ui;
 
 import 'package:flutter_web_plugins/flutter_web_plugins.dart';
 
-import 'generated_plugin_registrant.dart';
+import '$pluginRegistrantPath';
 import "${path.url.basename(buildStep.inputId.path)}" as entrypoint;
 
 Future<void> main() async {
@@ -384,6 +385,13 @@ Future<void> main() async {
     }
   }
 
+  /// Gets the relative path to the generated plugin registrant from the app
+  /// app entrypoint.
+  String _getPluginRegistrantPath(String entrypoint) {
+    return path.url.relative('lib/generated_plugin_registrant.dart',
+        from: path.url.dirname(entrypoint));
+  }
+
   @override
   Map<String, List<String>> get buildExtensions => const <String, List<String>>{
     '.dart': <String>['_web_entrypoint.dart'],

packages/flutter_tools/test/general.shard/build_runner/build_script_test.dart

@@ -69,6 +69,21 @@ void main() {
     verify(mockBuildStep.writeAsString(any,
         argThat(isNot(contains('registerPlugins(webPluginRegistry)'))))).called(1);
   });
+
+  test('FlutterWebShellBuilder correctly imports registrant', () async {
+    final AssetId nestedEntrypoint = AssetId('hello_world', 'lib/src/nested_main.dart');
+    when(mockBuildStep.inputId).thenReturn(nestedEntrypoint);
+
+    const FlutterWebShellBuilder builder = FlutterWebShellBuilder(
+      hasPlugins: true,
+      initializePlatform: true,
+    );
+
+    await builder.build(mockBuildStep);
+
+    verify(mockBuildStep.writeAsString(any,
+        argThat(contains("import '../generated_plugin_registrant.dart';")))).called(1);
+  });
 }
 
 class MockBuildStep extends Mock implements BuildStep {}