[ Widget Preview ] Update generated scaffold project to include early preview rendering (#162847)

With this change, `flutter widget-preview start` will launch a working
widget preview environment that can render previews from a target
project.

Also fixes an issue where `--offline` wasn't being respected by some pub
operations.

packages/flutter_tools/lib/src/base/signals.dart

@@ -124,7 +124,7 @@ class LocalSignals implements Signals {
 
     // If _handlersList[signal] is empty, then lookup the cached stream
     // controller and unsubscribe from the stream.
-    if (_handlersList.isEmpty) {
+    if (_handlersList[signal]!.isEmpty) {
       await _streamSubscriptions[signal]?.cancel();
     }
     return true;

packages/flutter_tools/lib/src/commands/widget_preview.dart

@@ -491,9 +491,26 @@ final class WidgetPreviewStartCommand extends WidgetPreviewSubCommandBase with C
     await pub.interactively(
       <String>[
         pubAdd,
+        if (offline) '--offline',
         '--directory',
         widgetPreviewScaffoldProject.directory.path,
-        '${rootProject.manifest.appName}:{"path":${rootProject.directory.path}}',
+        // Ensure the path using POSIX separators, otherwise the "path_not_posix" check will fail.
+        '${rootProject.manifest.appName}:{"path":${rootProject.directory.path.replaceAll(r"\", "/")}}',
+      ],
+      context: PubContext.pubAdd,
+      command: pubAdd,
+      touchesPackageConfig: true,
+    );
+
+    // Adds a dependency on flutter_lints, which is referenced by the
+    // analysis_options.yaml generated by the 'app' template.
+    await pub.interactively(
+      <String>[
+        pubAdd,
+        if (offline) '--offline',
+        '--directory',
+        widgetPreviewScaffoldProject.directory.path,
+        'flutter_lints',
       ],
       context: PubContext.pubAdd,
       command: pubAdd,

packages/flutter_tools/lib/src/widget_preview/preview_code_generator.dart

@@ -19,7 +19,7 @@ class PreviewCodeGenerator {
   /// project.
   final FlutterProject widgetPreviewScaffoldProject;
 
-  static const String generatedPreviewFilePath = 'lib/generated_preview.dart';
+  static const String generatedPreviewFilePath = 'lib/src/generated_preview.dart';
 
   /// Generates code used by the widget preview scaffold based on the preview instances listed in
   /// [previews].
@@ -43,10 +43,7 @@ class PreviewCodeGenerator {
   void populatePreviewsInGeneratedPreviewScaffold(PreviewMapping previews) {
     final Library lib = Library(
       (LibraryBuilder b) => b.body.addAll(<Spec>[
-        Directive.import(
-          // TODO(bkonyi): update with actual location in the framework
-          'package:widget_preview/widget_preview.dart',
-        ),
+        Directive.import('package:flutter/widgets.dart'),
         Method(
           (MethodBuilder b) =>
               b

packages/flutter_tools/templates/template_manifest.json

@@ -353,6 +353,10 @@
     "templates/plugin_swift_package_manager/macos.tmpl/projectName.tmpl/Sources/projectName.tmpl/PrivacyInfo.xcprivacy",
 
     "templates/widget_preview_scaffold/lib/main.dart.tmpl",
+    "templates/widget_preview_scaffold/lib/src/widget_preview_rendering.dart.tmpl",
+    "templates/widget_preview_scaffold/lib/src/controls.dart.tmpl",
+    "templates/widget_preview_scaffold/lib/src/generated_preview.dart.tmpl",
+    "templates/widget_preview_scaffold/lib/src/utils.dart.tmpl",
     "templates/widget_preview_scaffold/pubspec.yaml.tmpl",
     "templates/widget_preview_scaffold/README.md.tmpl",
 

packages/flutter_tools/templates/widget_preview_scaffold/lib/main.dart.tmpl

@@ -2,25 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// TODO(bkonyi): Implement.
+import 'src/widget_preview_rendering.dart';
+import 'src/generated_preview.dart';
 
-import 'package:flutter/material.dart';
-
-void main() {
-  runApp(const MainApp());
-}
-
-class MainApp extends StatelessWidget {
-  const MainApp({super.key});
-
-  @override
-  Widget build(BuildContext context) {
-    return const MaterialApp(
-      home: Scaffold(
-        body: Center(
-          child: Text('Hello World!'),
-        ),
-      ),
-    );
-  }
+Future<void> main() async {
+  await mainImpl(previewsProvider: previews);
 }

packages/flutter_tools/templates/widget_preview_scaffold/lib/src/controls.dart.tmpl

@@ -0,0 +1,102 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+
+class _WidgetPreviewIconButton extends StatelessWidget {
+  const _WidgetPreviewIconButton({
+    required this.tooltip,
+    required this.onPressed,
+    required this.icon,
+  });
+
+  final String tooltip;
+  final void Function()? onPressed;
+  final IconData icon;
+
+  @override
+  Widget build(BuildContext context) {
+    return Tooltip(
+      message: tooltip,
+      child: Ink(
+        decoration: ShapeDecoration(
+          shape: const CircleBorder(),
+          color: onPressed != null ? Colors.lightBlue : Colors.grey,
+        ),
+        child: IconButton(
+          onPressed: onPressed,
+          icon: Icon(
+            color: Colors.white,
+            icon,
+          ),
+        ),
+      ),
+    );
+  }
+}
+
+/// Provides controls to change the zoom level of a [WidgetPreview].
+class ZoomControls extends StatelessWidget {
+  /// Provides controls to change the zoom level of a [WidgetPreview].
+  const ZoomControls({
+    super.key,
+    required TransformationController transformationController,
+  }) : _transformationController = transformationController;
+
+  final TransformationController _transformationController;
+
+  @override
+  Widget build(BuildContext context) {
+    return Row(
+      mainAxisSize: MainAxisSize.min,
+      children: <Widget>[
+        _WidgetPreviewIconButton(
+          tooltip: 'Zoom in',
+          onPressed: _zoomIn,
+          icon: Icons.zoom_in,
+        ),
+        const SizedBox(
+          width: 10,
+        ),
+        _WidgetPreviewIconButton(
+          tooltip: 'Zoom out',
+          onPressed: _zoomOut,
+          icon: Icons.zoom_out,
+        ),
+        const SizedBox(
+          width: 10,
+        ),
+        _WidgetPreviewIconButton(
+          tooltip: 'Reset zoom',
+          onPressed: _reset,
+          icon: Icons.refresh,
+        ),
+      ],
+    );
+  }
+
+  void _zoomIn() {
+    _transformationController.value = Matrix4.copy(
+      _transformationController.value,
+    ).scaled(1.1);
+  }
+
+  void _zoomOut() {
+    final Matrix4 updated = Matrix4.copy(
+      _transformationController.value,
+    ).scaled(0.9);
+
+    // Don't allow for zooming out past the original size of the widget.
+    // Assumes scaling is evenly applied to the entire matrix.
+    if (updated.entry(0, 0) < 1.0) {
+      updated.setIdentity();
+    }
+
+    _transformationController.value = updated;
+  }
+
+  void _reset() {
+    _transformationController.value = Matrix4.identity();
+  }
+}

packages/flutter_tools/templates/widget_preview_scaffold/lib/src/generated_preview.dart.tmpl

@@ -0,0 +1,7 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/widgets.dart';
+
+List<WidgetPreview> previews() => <WidgetPreview>[];

packages/flutter_tools/templates/widget_preview_scaffold/lib/src/utils.dart.tmpl

@@ -0,0 +1,18 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+
+/// A basic vertical spacer.
+class VerticalSpacer extends StatelessWidget {
+  /// Creates a basic vertical spacer.
+  const VerticalSpacer({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const SizedBox(
+      height: 10,
+    );
+  }
+}

packages/flutter_tools/templates/widget_preview_scaffold/lib/src/widget_preview_rendering.dart.tmpl

@@ -0,0 +1,364 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter/services.dart';
+
+import 'controls.dart';
+import 'utils.dart';
+
+class WidgetPreviewWidget extends StatefulWidget {
+  const WidgetPreviewWidget({
+    super.key,
+    required this.preview,
+  });
+
+  final WidgetPreview preview;
+
+  @override
+  State<WidgetPreviewWidget> createState() => _WidgetPreviewWidgetState();
+}
+
+class _WidgetPreviewWidgetState extends State<WidgetPreviewWidget> {
+  final transformationController = TransformationController();
+  final deviceOrientation = ValueNotifier<Orientation>(Orientation.portrait);
+
+  @override
+  void initState() {
+    super.initState();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final previewerConstraints =
+        WidgetPreviewerWindowConstraints.getRootConstraints(context);
+
+    final maxSizeConstraints = previewerConstraints.copyWith(
+      minHeight: previewerConstraints.maxHeight / 2.0,
+      maxHeight: previewerConstraints.maxHeight / 2.0,
+    );
+
+    Widget preview = _WidgetPreviewWrapper(
+      previewerConstraints: maxSizeConstraints,
+      child: SizedBox(
+        width: widget.preview.width,
+        height: widget.preview.height,
+        child: widget.preview.child,
+      ),
+    );
+
+    preview = MediaQuery(
+      data: _buildMediaQueryOverride(),
+      child: preview,
+    );
+
+    preview = Column(
+      crossAxisAlignment: CrossAxisAlignment.start,
+      children: [
+        if (widget.preview.name != null) ...[
+          Text(
+            widget.preview.name!,
+            style: const TextStyle(
+              fontSize: 16,
+              fontWeight: FontWeight.w300,
+            ),
+          ),
+          const VerticalSpacer(),
+        ],
+        InteractiveViewerWrapper(
+          transformationController: transformationController,
+          child: preview,
+        ),
+        const VerticalSpacer(),
+        Row(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            ZoomControls(
+              transformationController: transformationController,
+            ),
+          ],
+        ),
+      ],
+    );
+
+    return Padding(
+      padding: const EdgeInsets.all(16.0),
+      child: Card(
+        child: Padding(
+          padding: const EdgeInsets.symmetric(
+            vertical: 16.0,
+            horizontal: 16.0,
+          ),
+          child: preview,
+        ),
+      ),
+    );
+  }
+
+  MediaQueryData _buildMediaQueryOverride() {
+    var mediaQueryData = MediaQuery.of(context);
+
+    if (widget.preview.textScaleFactor != null) {
+      mediaQueryData = mediaQueryData.copyWith(
+        textScaler: TextScaler.linear(widget.preview.textScaleFactor!),
+      );
+    }
+
+    var size = Size(widget.preview.width ?? mediaQueryData.size.width,
+        widget.preview.height ?? mediaQueryData.size.height);
+
+    if (widget.preview.width != null || widget.preview.height != null) {
+      mediaQueryData = mediaQueryData.copyWith(
+        size: size,
+      );
+    }
+
+    return mediaQueryData;
+  }
+}
+
+/// An [InheritedWidget] that propagates the current size of the
+/// WidgetPreviewScaffold.
+///
+/// This is needed when determining how to put constraints on previewed widgets
+/// that would otherwise have infinite constraints.
+class WidgetPreviewerWindowConstraints extends InheritedWidget {
+  const WidgetPreviewerWindowConstraints({
+    super.key,
+    required super.child,
+    required this.constraints,
+  });
+
+  final BoxConstraints constraints;
+
+  static BoxConstraints getRootConstraints(BuildContext context) {
+    final result = context
+        .dependOnInheritedWidgetOfExactType<WidgetPreviewerWindowConstraints>();
+    assert(
+      result != null,
+      'No WidgetPreviewerWindowConstraints founds in context',
+    );
+    return result!.constraints;
+  }
+
+  @override
+  bool updateShouldNotify(WidgetPreviewerWindowConstraints oldWidget) {
+    return oldWidget.constraints != constraints;
+  }
+}
+
+class InteractiveViewerWrapper extends StatelessWidget {
+  const InteractiveViewerWrapper({
+    super.key,
+    required this.child,
+    required this.transformationController,
+  });
+
+  final Widget child;
+  final TransformationController transformationController;
+
+  @override
+  Widget build(BuildContext context) {
+    return InteractiveViewer(
+      transformationController: transformationController,
+      scaleEnabled: false,
+      child: child,
+    );
+  }
+}
+
+// TODO(bkonyi): according to goderbauer@, this probably isn't the best approach to ensure we
+// handle unconstrained widgets. This should be reworked.
+/// Wrapper applying a custom render object to force constraints on
+/// unconstrained widgets.
+class _WidgetPreviewWrapper extends SingleChildRenderObjectWidget {
+  const _WidgetPreviewWrapper({
+    super.child,
+    required this.previewerConstraints,
+  });
+
+  /// The size of the previewer render surface.
+  final BoxConstraints previewerConstraints;
+
+  @override
+  RenderObject createRenderObject(BuildContext context) {
+    return _WidgetPreviewWrapperBox(
+      previewerConstraints: previewerConstraints,
+      child: null,
+    );
+  }
+
+  @override
+  void updateRenderObject(
+    BuildContext context,
+    _WidgetPreviewWrapperBox renderObject,
+  ) {
+    renderObject.setPreviewerConstraints(previewerConstraints);
+  }
+}
+
+/// Custom render box that forces constraints onto unconstrained widgets.
+class _WidgetPreviewWrapperBox extends RenderShiftedBox {
+  _WidgetPreviewWrapperBox({
+    required RenderBox? child,
+    required BoxConstraints previewerConstraints,
+  })  : _previewerConstraints = previewerConstraints,
+        super(child);
+
+  BoxConstraints _constraintOverride = const BoxConstraints();
+  BoxConstraints _previewerConstraints;
+
+  void setPreviewerConstraints(BoxConstraints previewerConstraints) {
+    if (_previewerConstraints == previewerConstraints) {
+      return;
+    }
+    _previewerConstraints = previewerConstraints;
+    markNeedsLayout();
+  }
+
+  @override
+  void layout(
+    Constraints constraints, {
+    bool parentUsesSize = false,
+  }) {
+    if (child != null && constraints is BoxConstraints) {
+      double minInstrinsicHeight;
+      try {
+        minInstrinsicHeight = child!.getMinIntrinsicHeight(
+          constraints.maxWidth,
+        );
+      } on Object {
+        minInstrinsicHeight = 0.0;
+      }
+      // Determine if the previewed widget is vertically constrained. If the
+      // widget has a minimum intrinsic height of zero given the widget's max
+      // width, it has an unconstrained height and will cause an overflow in
+      // the previewer. In this case, apply finite constraints (e.g., the
+      // constraints for the root of the previewer). Otherwise, use the
+      // widget's actual constraints.
+      _constraintOverride = minInstrinsicHeight == 0
+          ? _previewerConstraints
+          : const BoxConstraints();
+    }
+    super.layout(
+      constraints,
+      parentUsesSize: parentUsesSize,
+    );
+  }
+
+  @override
+  void performLayout() {
+    final child = this.child;
+    if (child == null) {
+      size = Size.zero;
+      return;
+    }
+    final updatedConstraints = _constraintOverride.enforce(constraints);
+    child.layout(
+      updatedConstraints,
+      parentUsesSize: true,
+    );
+    size = constraints.constrain(child.size);
+  }
+}
+
+/// Custom [AssetBundle] used to map original asset paths from the parent
+/// project to those in the preview project.
+class PreviewAssetBundle extends PlatformAssetBundle {
+  // Assets shipped via package dependencies have paths that start with
+  // 'packages'.
+  static const String _kPackagesPrefix = 'packages';
+
+  @override
+  Future<ByteData> load(String key) {
+    // These assets are always present or are shipped via a package and aren't
+    // actually located in the parent project, meaning their paths did not need
+    // to be modified.
+    if (key == 'AssetManifest.bin' ||
+        key == 'AssetManifest.json' ||
+        key == 'FontManifest.json' ||
+        key.startsWith(_kPackagesPrefix)) {
+      return super.load(key);
+    }
+    // Other assets are from the parent project. Map their keys to those found
+    // in the pubspec.yaml of the preview envirnment.
+    return super.load('../../$key');
+  }
+
+  @override
+  Future<ImmutableBuffer> loadBuffer(String key) async {
+    return await ImmutableBuffer.fromAsset(
+      key.startsWith(_kPackagesPrefix) ? key : '../../$key',
+    );
+  }
+}
+
+/// Main entrypoint for the widget previewer.
+///
+/// We don't actually define this as `main` to avoid copying this file into
+/// the preview scaffold project which prevents us from being able to use hot
+/// restart to iterate on this file.
+Future<void> mainImpl({
+  required List<WidgetPreview> Function() previewsProvider,
+}) async {
+  runApp(
+    _WidgetPreviewScaffold(
+      previewsProvider: previewsProvider,
+    ),
+  );
+}
+
+class _WidgetPreviewScaffold extends StatelessWidget {
+  const _WidgetPreviewScaffold({required this.previewsProvider});
+
+  final List<WidgetPreview> Function() previewsProvider;
+
+  @override
+  Widget build(BuildContext context) {
+    final List<WidgetPreview> previewList = previewsProvider();
+    Widget previewView;
+    if (previewList.isEmpty) {
+      previewView = const Column(
+        mainAxisAlignment: MainAxisAlignment.center,
+        children: <Widget>[
+          Center(
+            // TODO: consider including details on how to get started
+            // with Widget Previews.
+            child: Text(
+              'No previews available',
+              style: TextStyle(color: Colors.white),
+            ),
+          )
+        ],
+      );
+    } else {
+      previewView = LayoutBuilder(
+        builder: (BuildContext context, BoxConstraints constraints) {
+          return WidgetPreviewerWindowConstraints(
+            constraints: constraints,
+            child: SingleChildScrollView(
+              child: Column(
+                mainAxisAlignment: MainAxisAlignment.center,
+                children: <Widget>[
+                  for (final WidgetPreview preview in previewList) WidgetPreviewWidget(preview: preview),
+                ],
+              ),
+            ),
+          );
+        },
+      );
+    }
+    return MaterialApp(
+      debugShowCheckedModeBanner: false,
+      home: Material(
+        color: Colors.transparent,
+        child: DefaultAssetBundle(
+          bundle: PreviewAssetBundle(),
+          child: previewView,
+        ),
+      ),
+    );
+  }
+}

packages/flutter_tools/test/commands.shard/permeable/utils/project_testing_utils.dart

@@ -7,6 +7,7 @@ import 'dart:convert';
 import 'package:flutter_tools/src/artifacts.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/base/io.dart';
+import 'package:flutter_tools/src/cache.dart';
 import 'package:flutter_tools/src/globals.dart' as globals;
 import 'package:process/process.dart';
 
@@ -47,7 +48,7 @@ Future<void> analyzeProject(
   List<String> expectedFailures = const <String>[],
 }) async {
   final String flutterToolsSnapshotPath = globals.fs.path.absolute(
-    globals.fs.path.join('..', '..', 'bin', 'cache', 'flutter_tools.snapshot'),
+    globals.fs.path.join(Cache.flutterRoot!, 'bin', 'cache', 'flutter_tools.snapshot'),
   );
 
   final List<String> args = <String>[flutterToolsSnapshotPath, 'analyze'];

packages/flutter_tools/test/commands.shard/permeable/widget_preview_test.dart

@@ -31,11 +31,12 @@ void main() {
   late LoggingProcessManager loggingProcessManager;
   late FakeStdio mockStdio;
   late Logger logger;
-  late FileSystem fs;
+  late LocalFileSystem fs;
   late BotDetector botDetector;
   late Platform platform;
 
-  setUp(() {
+  setUp(() async {
+    await ensureFlutterToolsSnapshot();
     loggingProcessManager = LoggingProcessManager();
     logger = BufferLogger.test();
     fs = LocalFileSystem.test(signals: Signals.test());
@@ -43,10 +44,16 @@ void main() {
     tempDir = fs.systemTempDirectory.createTempSync('flutter_tools_create_test.');
     mockStdio = FakeStdio();
     platform = FakePlatform.fromPlatform(const LocalPlatform());
+    // Most, but not all, tests will run some variant of "pub get" after creation,
+    // which in turn will check for the presence of the Flutter SDK root. Without
+    // this field set consistently, the order of the tests becomes important *or*
+    // you need to remember to set it everywhere.
+    Cache.flutterRoot = fs.path.absolute('..', '..');
   });
 
   tearDown(() {
     tryToDelete(tempDir);
+    fs.dispose();
   });
 
   Future<Directory> createRootProject() async {
@@ -91,11 +98,13 @@ void main() {
     final Directory widgetPreviewScaffoldDir = widgetPreviewScaffoldFromRootProject(
       rootProject: rootProject ?? fs.currentDirectory,
     );
-    expect(widgetPreviewScaffoldDir, exists);
-    expect(
-      widgetPreviewScaffoldDir.childFile(PreviewCodeGenerator.generatedPreviewFilePath),
-      exists,
-    );
+    // Don't perform analysis on Windows since `dart pub add` will use '\' for
+    // path dependencies and cause analysis to fail.
+    // TODO(bkonyi): enable analysis on Windows once https://github.com/dart-lang/pub/issues/4520
+    // is resolved.
+    if (!platform.isWindows) {
+      await analyzeProject(widgetPreviewScaffoldDir.path);
+    }
   }
 
   Future<void> cleanWidgetPreview({required Directory rootProject}) async {
@@ -181,13 +190,12 @@ void main() {
     );
 
     const String samplePreviewFile = '''
-// This doesn't need to be valid code for testing as long as it has the @Preview() annotation
 @Preview()
-WidgetPreview preview() => WidgetPreview();''';
+WidgetPreview preview() => const WidgetPreview(child: Text('Foo'));''';
 
     const String expectedGeneratedFileContents = '''
 // ignore_for_file: no_leading_underscores_for_library_prefixes
-import 'package:flutter_project/foo.dart' as _i1;import 'package:widget_preview/widget_preview.dart';List<WidgetPreview> previews() => [_i1.preview()];''';
+import 'package:flutter_project/foo.dart' as _i1;import 'package:flutter/widgets.dart';List<WidgetPreview> previews() => [_i1.preview()];''';
 
     testUsingContext(
       'start finds existing previews and injects them into ${PreviewCodeGenerator.generatedPreviewFilePath}',

packages/flutter_tools/test/general.shard/widget_preview/preview_code_generator_test.dart

@@ -25,7 +25,7 @@ void main() {
       final Directory projectDir =
           fs.currentDirectory.childDirectory('project')
             ..createSync()
-            ..childDirectory('lib').createSync();
+            ..childDirectory('lib/src').createSync(recursive: true);
       project = FlutterProject(projectDir, manifest, manifest);
       codeGenerator = PreviewCodeGenerator(widgetPreviewScaffoldProject: project, fs: fs);
     });
@@ -56,7 +56,7 @@ void main() {
         // The generated file is unfortunately unformatted.
         const String expectedGeneratedPreviewFileContents = '''
 // ignore_for_file: no_leading_underscores_for_library_prefixes
-import 'foo.dart' as _i1;import 'src/bar.dart' as _i2;import 'package:widget_preview/widget_preview.dart';List<WidgetPreview> previews() => [_i1.preview(), _i2.barPreview1(), _i2.barPreview2(), ];''';
+import 'foo.dart' as _i1;import 'src/bar.dart' as _i2;import 'package:flutter/widgets.dart';List<WidgetPreview> previews() => [_i1.preview(), _i2.barPreview1(), _i2.barPreview2(), ];''';
         expect(generatedPreviewFile.readAsStringSync(), expectedGeneratedPreviewFileContents);
 
         // Regenerate the generated file with no previews.
@@ -69,7 +69,7 @@ import 'foo.dart' as _i1;import 'src/bar.dart' as _i2;import 'package:widget_pre
         // - An import of the widget preview library
         // - A top-level function 'List<WidgetPreview> previews()' that returns an empty list.
         const String emptyGeneratedPreviewFileContents = '''
-import 'package:widget_preview/widget_preview.dart';List<WidgetPreview> previews() => [];''';
+import 'package:flutter/widgets.dart';List<WidgetPreview> previews() => [];''';
         expect(generatedPreviewFile.readAsStringSync(), emptyGeneratedPreviewFileContents);
       },
     );